A possible solution could be to add a timer on weapon (class) swapping.
I think it's easy to over-complicate things. People who have played FFXI as their MMO of choice are accustomed to regular weapon swapping. They want to preserve that concept because they enjoyed it. Personal preference is personal preference. People who have played MMOs where swapping weapons (or any gear, for that matter) in the middle of a fight is not allowed may not see what all the fuss is about. They've learned how to play a game where the gear you're wearing when you start the fight is always the gear you're wearing when the fight is over and that's that.
To a lot of people, gear swapping mid-combat is a clunky mechanic. It's a response to horizontal progression. If you've got the best you can hope to get for your job's role in a general sense, what is there left to strive for for the next 3 years? Uber-grind demigod bits and...offset pieces. Yes. The carrot dangling to the side is adequately enticing in the absence of a carrot in front of you, so off we go! Of course, that inevitably leads to complaints about inventory. The developers likely didn't intend for its players to be carrying around 2/4/8 different weapons and multiple doodads and armor bits to be swapped around in combat. And since most gear swaps were automated by macro associated with a particular spell/ability, it only added the illusion of depth. I can cut and paste a macro like nobody's business and once I've done so, the actual act of triggering that macro is no more complicated whether the macro contains 2 gear swaps and an ability or a simple taru leg humping emote. Yay I've got the gear. Yay I've got the macro. I'm fooling myself if I think it means much of anything.
Of course the simple, objective solution to all of this is that when SE says no weapon swapping mid-combat, they mean no weapon swapping mid-combat and to have everyone accept it at face value. Since we know that we'll be able to change weapons in the field in order to change classes it makes sense that some inventory accommodations will be made to account for it. Start throwing in multiple swappable bits for multiple classes and I think the inventory demands would get out of hand pretty quickly. I also think that in a more involved and dynamic combat setting, people aren't going to miss swapping gear. They'll be too busy doing other things to really think about it.
It's too early to start proposing changes to a combat system we haven't seen in detail yet in order to preserve a dated combat system that I think a lot of people would prefer to largely do away with. It might be better to take a close look at what SE is offering before we round up the posse and start pushing for design adjustments to accommodate an aversion to change.