It sounds like you are trying to invent prolander again which is probably the most failed competitive format in this game and is not currently being run because nobody wants to play it. So pragmatically at least I imagine most ppl will dismiss this because it's already a time proven way of killing the game. Nonetheless though I think these discussions are important (as long as ppl argue in good faith) so I'll engage and offer my opinion as somebody who mains a dogshit class (pyro) and has way more hl experience than 6s.
TynnyriI understand completely, learning new skills and adapting to a changing landscape might not always be fun. But that is the heart of competition, and that's where fun should be derived in a competitive setting. Any "fun" that is derived from no need to learn, adapt or improve isn't competitive.
This is... a strange view to say the least. The need to learn or adapt should be correlated with competitiveness but it's not the end goal. I can make 100 dogshit changes to any format that will definitely force players to adapt and change but I think most people would agree that that makes things less competitive, not more. For FPS games and a lot of esports in general execution (or skill expression) is just as important for the competitive aspect and is the reason why a lot of people watch or play it. Your ability to adapt and change is conditional on your execution skills and both are needed.
TynnyriHowever, the consensus in 6v6 seems to be that allowing heavies or pyros to even be viable (which is the sole reason given for Fists of Steel, Detonator, and Scorch Shot bans), will eventually lead to a slow, boring meta. But that makes no sense. Highlander seems fine with these weapons. Sniper can easily kill Heavies, and Pyros lose against all hitscan weapons. So why can't a team break through a stalemate by switching classes to, for example, Sniper? (who counters both Pyro and Heavy) Most issues with stalemates seem to be caused by 5CP's brittle balance, but if that was the sole reason, people would've banned 5CP. So these balance issues seem to go deeper, affecting even KOTH and stopwatch.
I assume you're EU because pyro unlocks aren't banned in NA. But I think you're mistaken here, people really want to play 5cp so they would not have banned it, just ban the stuff that makes it awful for 5cp. The highlander solution is to ban 5cp (again in NA I don't follow EU HL) because as you alluded to it is so awful once you have full time defensive classes.
TynnyriThink of it this way: Would it be healthy for the game if we could stack Medics? Of course not, more players would be forced into over-centralized, Medic-focused meta. Everyone should agree that stacking a class creates issues, mainly of over-centralization.
Over-centralization is not the core reason that stacking meds would be bad though. No matter what tf2 competitive format you play you already play in an incredibly medic-centralized metagame. It's bad for direct gameplay reasons (you need to be able to kill medics to allow the game to flow), not some higher level metagaming reasons. You can argue that in theory it would be nice to not have the game so centralized around one class and that's a perfectly fine argument, but how the platonic ideal of a game should look like doesn't really have much bearing on how it actually plays out unfortunately and should not be the primary reason of how you balance the actual game.
TynnyriIf players were FORCED to pick specialists via a class limit of 1, this problem should be solved. I am not saying it will be more fun for the Scout players, but it would bring more competitiveness to the game.
I've already talked about why that's a weird definition of competitiveness but I also want to make a point here; oftentimes it's also just not fun for the people maining the dogshit classes like pyro. This is actually a very common highlander phenomenon; people who main stuff like pyro/engineer want to play comp for the first time and naturally gravitate toward HL cause you can't play it in 6's without throwing. But then they quit highlander because the gameplay on those classes is and will never be anywhere near pubs. Going from doing random shit in pubs on engie and then forced to sit on your sniper in HL is quite jarring. Yes the format of 6s makes certain classes unviable full time, but the formats that do allow for those classes to be played are still not even close to pubs. No amount of format trickery can save a class that is just inherently bad. So really when pubbers claim that 6s sucks because they can't play their favorite class, the reality is that sorry, the version of the class that you want to play doesn't really exist in any competitive format.
TynnyriPunishing Soldier's mistakes when he can easily reposition himself is harder compared to punishing Pyro's or Heavy's mistakes, when they can't.
Weird way of phrasing it, it's not really a mistake by the soldier player then if they have an escape option. Similar to how on pyro you can linger against uber longer than many classes because you have airblast, that's not a mistake of the pyro player. What constitutes a mistake is a function of the tools your class has, who cares about the fact that you would be dead if you were playing a different class.
TynnyriClass' exploitable weaknesses are a great way to enforce skill expression. Kind of like Highlander, Spy: The weakest class - But that's what makes a good spy player so important, to be able to turn a low kill count into something more.
Spy is not the weakest class in HL, probably around 6th (med demo sniper scout soldier higher, some people might actually swap spy and soldier or say heavy better than spy).