Ehh I think it's a bit more than simply being the general mentality.
A heavy is simply too hard to push against, and there's no other reliable way to counter it other than mirroring. Even though his mobility may be detrimental, at all times one team is trying to hold ground and one is trying to gain it. Speed is a non issue for the holders, so they'll run a heavy, so then the pushers need a heavy to take down their heavy, and yay everyone's running heavy all the time. Dump a heavy on your point on sixties and bam, they can't push you off without one too. Honestly I think banning him would really diversify class selection, giving place for classes like deto and pyro. But then we have the whole concept of completely banning classes in a class based game which is kind of durr.
Soldier on the other hand is played because it's easy (not to devalue good soldiers or justify bad ones). I'd definitely like to see pyro being run instead of solly, especially with his buffs, and deto with the damage buff could be played in place as well. I guess the theory behind running deto or pyro is if you're really good you can outperform a soldier. I unno if that actually holds true, and it's kind of difficult to measure with no demo system, but the possibility is there.
It'd actually be really cool if we could get Flow to break the soldier mentality and bring some classes to be 'full time' replacements.
And for scout I think it boils down to two things. 1, we picked the shittiest game mode to be our most popular. ctf just makes scout way too powerful, and we really only have one good map, yet we play it the most. :/
And 2, although on ctf scout's speed should mean it's easier for him to offclass since he can run to spawn faster, the offclasses aren't really effective enough for people to justify learning totally new classes and when they apply etc. It's basically become a lost art. Again it would be cool if Flow could reintroduce it. As game modes go, 3cp and koth are far better for increasing the viability of other classes.
but yeah basically what I'm saying is the standard rollout is more easymodo