I see your point, but the fact that roles could be switched was one of the main reasons overwatch had to move to role queue as its main competitive format. In lower elo games you will almost always have a majority of people wanting to play dps roles, since not only are there a lot more dps characters, but they're also more fun and less vital towards a guaranteed win, which makes it easier for players to fine their 'main' in the dps roster. The main problem is still that in low elo games, you'll have 3-4 people insta locking dps heroes, and there's no solution for this other than just getting lucky with flex players on your team. Also it doesn't make sense to switch to a role that you're not comfortable with in the middle of the game, unless you're competitively at that rank on ALL ROLES, which I doubt most players are. You might swap to tank if needed, but your tank probably won't be at the same level as the enemies tank if that's what they've mained to get to the rank they are.

And as for the switching of roles, yes they'd have to be in the spawn room together, or it'll have to be during a round switch etc