kooturtlewords
As a player that experienced low level pugs and climbed my way out of them, I can say that the reason why it was a good thing to not have offclasses full time is that they are just worse classes. A team trying to push with a pyro and a sniper into last with uber advantage can be very difficult. Even missing 1 scout or 1 soldier can make it much more difficult to push doors, especially into last. It becomes apparent very quickly that its literally trolling to keep offclassing like that, but low level players may know that its very cringe instinctively. I bet if you took a team that rolled out to process mid on sniper, pyro, heavy against an advanced team, they could just cap the point in your face nearly every time because these classes lack mobility or range.
Obviously no one cares about offclassing on last, but offclassing on second and mid is just worse almost all of the time. The only exceptions are sniper and spy. Obviously low-level pug snipers aren't going to be very effective, and if people play spy all the time, it is no longer effective, therefore it generally goes back to cookiecutter being the standard.
Low level players don't often know how to exploit the disadvantages of having these classes, but you can still bet that the team with the cookie cutter classes will win mid more often than the offclass team. If they can win mid more, they will generally win more. It is not a question why they teach them cookie cutter. It is because it will win them games. TF2 isn't really a game of class counters on 5CP. You play the most damaging, fastest, and highest ranged characters if you want to win and press your advantages.
If you are at a disadvantage, then the offclasses can be used effectively in very clear criteria. The offclasses themselves other than sniper don't really require any skill to be used effectively. A heavy or engineer setting up on last, a pyro airblasting the uber on maps like gullywash, a spy backstabbing a medic, or even a sniper sitting in spawn watching an angle, these all don't require much teaching or learning to be effective.
It isn't like I or most players still playing today discovered that this class composition is very effective. TF2 players from the stone age of competitive realized that it is powerful and fun to play, because soldier, demo, and scout are actually very full of skill expression, and very good at killing enemy players without also blowing up.