Is that acidrenix? oh baby.
I'll post in response to you not disagreeing but stating that your argument is too well thought out for something like this.
I can almost guarantee this mode never gets large. It's fun, but it's random. There's a reason why we have weapon bans in TF2 6v6, there's a reason we limit classes to 1 medic 1 demo, and there's a reason every season we talk about map pools and new whitelists and rulesets (ESEA SHOULD MOVE TO THE EURO MATCH TIMELIMIT/RULES IMO).
It's that tf2 by itself isn't a balanced game. You can theorycraft all sorts of strats for this 5v5 AR mode but at the end of the day it might as well be All Random and not Arena Respawn. You can't plan for weapons on the other team at all, one round a pyro with degreaser shotgun axe and the next round natascha heavy, there's actually no way you can predict a round or organize a flawless rollout, or counterplay because literally you may as well be thrown in a blender every round.
If tf2 actually wants to be esports viable, which arguably it isn't in its current state due to the fact that pubbing and 6v6 are COMPLETELY different, it's going to need matchmaking and it's going to need matchmaking that is 6v6 and somewhat restricted weaponwise. We'll see what happens though, that's up to Valve.
AR is a cool idea, but again, this game needs weapon bans to be legitimate spectator-wise. You can't cast a game with no whitelist. "HOLD ON LET ME CHECK HIS LOADOUT" would be 90% of the cast.