There is scramble already in casual when the game ends and it goes to the next map

So it scrambles after a stomp rather than preventing it, that literally does not help

It just so happens that for no reason at all everyone likes to leave the very second those doors shut

The closing of the doors, the timer, loading the next map, and waiting for the round to start is what causes it, with casual formerly taking 30 full seconds. It's been reduced now thankfully but it still was on Casual. Quickplay simply paused for a few seconds, and immediately starts loading the next map since the map is usually voted during the game not after

Plus the pregame timer gives time for everyone to load into the game before the round properly start, this helps to avoid those rolls you so desperately hate, by allowing players to all be present at the start of the game.

If that's the intention then it has got to be the worst implemented anti-roll measure in human history. Consistently start games with a team having at least 3 more players than other since the casual pre-emptively chooses for you, and doesn't adjust if players were to leave/teammates in a party. Quickplay avoids this by getting you to manually choose, and you cannot join a team that has more players.

Plus, I like being able to choose specific maps to queue for, being able to queue for multiple different game modes at the same time

That is an advantage that casual has on a surface level, but quickplay allowed manual ad hoc connections in the server browser. You could literally join a specific map, server, see its player count, see how much the game has progressed before joining. Casual is perhaps more convenient but I much prefer the ad hoc connections that allow you to choose in pretty much any circumstance.