well yeah, and all the good players instantly switching wasn't my experience on quickplay. and even though it sometimes happened, systems could have very well been put into place to make it harder.
and anyway, teamstackers won't get separated from eachother in the first place in casual mode, because they're in a party.
Teamstacker seems like a poor word choice there, it seems you're just talking about people being in a party. I'd define stacking teams as what happens when a good player speculates on which team is going to win and switches to it, which is impossible in casual (from what people have said here anyway, I haven't tried it). Also, autobalance breaking up actively stacked teams never happened, because a person could join spectator and wait a couple minutes to switch to the winning team again.
One thing that I think would bother me about casual (though I'll probably never try it while the server browser exists, I never
trusted quickplay either) would be not being able to switch onto the losing team. Those are the fun matches, when you get to actually turn things around, and as much as people complained about getting autobalanced, I think I almost always ended up glad when it happened to me. Seeing your teammates winning with their minisentries and heavies and plastic-tier puff pyros,
so cock sure, and not being able to shit in their milkshake, that would drive me crazy.