Offclasses just make the flow of the game sooo much worse, especially pushing lasts. A lot of time in stalemates is used just trying to take down a gun which is incredibly hard because when you try and pressure the choke the engi’s teammates are just going to spam or snipe you, not to mention he is repairing his gun all the time or might have a pyro. You could actually take some phat ubers if sniper didn’t exist also but you have to pop earlier than you would have so you don’t get headshot.
If these classes didn’t exist I think the game would become way more skillful and less stalematey because everyone on your team would have to have good positioning and health management to adapt to their strat and punish the other team. Right now teams don’t have to risk anything because they can just stand back with a gun which you can’t spam before the push because you will get headshot.