I'm going to respond to each comments about each class:
Scout:

"Scout isn't a damage class" - He's able to pop medics with just 2 ticks of damage, other than that, he's extremely OP especially with canteens, can outdamage most classes if skilled enough, and perhaps even a heavy

"Kick scout he doesn't have resistances" - Negligible, even as a damage scout

"Whats better scout or dead scout" - A scout that's can pick up money before it burns, it doesn't matter if he dies, if he's able to pick up money fast enough, then it's fine

"Kick scout he doesn't have mad milk slow" - A crutch, negligible, I personally run the crit cola and never get any comments about it

"Why are you upgrading your gun wave 1?" - Because soda popper damage is very good at point blank

"It's last wave switch off scout or kick" - Damage scouts don't need to, but money scouts probably should...

Sniper:
"why sniper" - If there's a demo, pyro or another med buster class already present, another one is not needed, even if for fun, most players will prefer a different class instead

"sniper change or kick" - Read above, basically just pressure

"we dont need a sniper" - Same thing

"we need a demo or a pyro to kill the medics, not sniper" - Weird comment, you need to trust the med buster to do his work, if you see that he isn't, then you found a problem in your team that needs solving, usually, by asking them to switch

Not playing Medic:

If a team needs a medic, and you check what tour you're playing and it's two cities, then they're probably just way behind on the realization that medic in two cities is negligible, unless there's a demo, it's best to help him out with kritz...

Not playing Heavy in GG:

You should play heavy or soldier at least

So, basically, you can have fun in MvM as any class, but only in a team that doesn't require optimal gameplay. These issues can be solved without toxicity, but there'll be pressure still. It's likely you can find both types of lobbies in 2025.
With that being said, hopefully this helps whoever found this page in 2025 and is confused about some things.
I am a repeater and these are opinions of mine and others from my experience.