While I don’t think there are many players that will share the concers that the OP posts, under his point of view and the way he plays the game, his request makes sense and I do feel for the OP if this is something he has been working on for years and gets broken by the warband system.
At the same time this seems like a very niche case that I’m not sure the Devs will or should account for, you simply can’t please everyone and more account-wide stuff is something the overwhelming majority of players want.
I guess if it wasn’t too difficult from a techincal standpoint it would be ok to provide the option to opt-out but I really really doubt that will be the case, it sounds like this is a signifficant change/rework at a game-systems level that will probably require some sort of “migration” tools to collect all the indivudual things for each character and convert it into the new warbard system.
My suggestion or I think the approach I’d go with if I was in the OP’s place is to try to see if there’s maybe an addon or something that can do what he’s asking to (like keeping track of inidividual achievements).