Let me first clarify something. Item destruction is a broad term for the many ways in which items can be destroyed. Decay is a specific version of that, a way that items can loose durability and eventually be destroyed. This is an important distinction. It's the difference between the economic models in something like EVE online compared to SWG. In other words, not all item destruction systems use decay specifically. Going forward, I will continue to respond to your mentions of "decay" as though we are speaking about item destruction in general.
As I said, the primary purpose of item destruction (worn or carried items in particular) is it's economic value as a sink, that remains true whether the progression system is horizontal or vertical. (Technically, repairs can be effective enough sinks if made to be, full on destruction of the item is to ensure that it has to be re-created and/or re-bought. That goes beyond just a sink and into supply and demand)
As long as it is understood that the primary motivator's for play will come from things other than simply-replacing-the-gun-you-already-had we are on the same general page. As far as what variety of things actually do serve as effective motivation for players, that is honestly it's own long conversation. "better" and/or "new" things are the most obvious answers, but that is hardly a complete list.
At no point has the argument been that Em-8Er doesn't need a sink. It has all been a question of what those sinks should be. With that established, keep in mind that Em-8ER puts more at the players finger tips than just the gear they wear and carry. Vehicles, bases and other group goals will serve as a heavy resource sink. That was an element that firefall barely touched on, and it makes a world of difference in how much of a sink worn gear actually has to be. On that note, once we start looking at things like bases being built and potentially destroyed, we open up a variety of new gameplay options that are for more capable of greatly altering moment to moment experiences in a way that players are likely to respond positively to.
To truly dig into a conversation about economic model requires a lot of specifics and nailing down how carious nuances would be handled. Is it possible to make a system where items permanently break but can then be replaced with a simple button press? Certainly, if it is built from the ground up for that very purpose. Does the need to replace items that brake offer a basis for constant supply and demand? Certainly, but only if the entire system is built to ensure that other things won't impede that process. Can an economic model be built in a way that a player can be a full time crafter? Potentially yes, but that would be assuming that kind of playstyle was even a goal of the game in question. As of yet, there has been no announcement that Em-8ER is intending to offer such a playstyle.
Let me address one thing in particular.
That's another one of those myths about what item destruction does for a game. In terms of cause and effect, why would that be true? An item breaking isn't actually removing it's balance issues from the game. As long as players have the knowledge and the means (In other words the recipe and resources) to make the item in question, they will simply re-make the item as the old one breaks. To remove it, the devs have to either need the recipe or make the required resources unatainable. Besides, is there some reason to believe that players would respond any better to their OP item breaking than their OP item being nerfed in a patch? The result is the same.
That digresses away from the issue of economic model and delves into the complex discussion of horizontal verses vertical progression models. It also opens up the discussion about whether quality vs quanity makes any difference in terms of work load on a developer. I'd like to leave those issues for another conversation.
I don't want to shortchange the depth of the conversation about why people stick with an mmo and why they don't. Beyond that, to nail down exactly what was at play that made firefall special and what it was that made it unable to reach and retain a player base size capable of creating a return on it's investment is one heck of a long and complex conversation. I am going to leave those lines of discussion alone for now. All I will say, is that different people stuck with it for different reasons.
I should have written this more precise - by decay I meant decay in game with horizontal progression (it may have some constant vertical progression).
As long as it is understood that the primary motivator's for play will come from things other than simply-replacing-the-gun-you-already-had we are on the same general page. As far as what variety of things actually do serve as effective motivation for players, that is honestly it's own long conversation. "better" and/or "new" things are the most obvious answers, but that is hardly a complete list.
This kind of games must have sinks. If not, after few months everyone will have tons of useless and worthless resources. I don't know any better sink than repairing and decay.
To truly dig into a conversation about economic model requires a lot of specifics and nailing down how carious nuances would be handled. Is it possible to make a system where items permanently break but can then be replaced with a simple button press? Certainly, if it is built from the ground up for that very purpose. Does the need to replace items that brake offer a basis for constant supply and demand? Certainly, but only if the entire system is built to ensure that other things won't impede that process. Can an economic model be built in a way that a player can be a full time crafter? Potentially yes, but that would be assuming that kind of playstyle was even a goal of the game in question. As of yet, there has been no announcement that Em-8ER is intending to offer such a playstyle.
Let me address one thing in particular.
it is the best tool to remove overpower gear from the game without direct intervention and players rage,
In typical MMORPG most players skip low level instances (low exp, too much time for complete, low level rewards, hard to find party). It is easier and faster go to another zone and do higher level "kill X mobs". All that instances are wasted content. New patch must be new endgame zone with new endgame instances. In Firefall old content was mixed with new one and it was playable all the time, so no content was wasted. Horizontal games still need new content, but with higher quality than quantity.
About player motivation to play. Most MMOs force players to grind - it is the simplest goal that everyone understands. After reaching maximum level, most players leave the game until next big patch. They don't feel connection with game - it is another MMO, so they jumping between them.
Firefall achieved much more - players that were bound to game even when it was changed into crappy WoW-clone. What's more, players actively played Firefall, when it had very small content (few events, thumping, fighting with non-stop spawning Chosens). I still ask myself "Why have I left Firefall so late?" and I don't know the answer. Even after Firefall died, we still have hope, that Em-8ER will be reincarnation of original Firefall. This kind of bound is much better motivation than grind goals.
Firefall achieved much more - players that were bound to game even when it was changed into crappy WoW-clone. What's more, players actively played Firefall, when it had very small content (few events, thumping, fighting with non-stop spawning Chosens). I still ask myself "Why have I left Firefall so late?" and I don't know the answer. Even after Firefall died, we still have hope, that Em-8ER will be reincarnation of original Firefall. This kind of bound is much better motivation than grind goals.