So, your argument from the very beginning is that unless something is expressly forbidden/mandated, then it's not an actual "rule" of the game. One of the examples you use is that the game does not literally require you, on the level of the game engine, to touch the cart during overtime.

So let's go back to the tic-tac-toe example. There is nothing that is expressly mandated or forbidden about a game of tic-tac-toe. It's not like the laws of physics are somehow going to prevent you from drawing green diamonds everywhere, even over the lines separating the spaces. This is the same as in your example of not touching the cart in overtime.

The problem comes in when we start thinking about what makes a game a game and the answer to that question is "arbitrary rules". Tic-tac-toe only works when both you and I agree to only draw EXs and Ohs (Goddammit Elle King...) in the nine given spaces. If we don't do that, then we might be playing something, but it's not tic-tac-toe.

Now, Overwatch is a team shooter with specific roles that create strong teams. While it's true you're not obligated to play a role that rounds out your team, or play in a way that compliments your team, if you don't, then you're not playing the game the way it is intended. In fact, Blizzard explicitly avoided including things like direct k/d ratios because they wanted to encourage team play instead of individual prowess. So just like in the tic-tac-toe example where you can draw whatever shapes you want, wherever you want using the same engine (reality is an engine now), in Overwatch when you run out and try to solo everything, ignore what your team needs and just do it all entirely for yourself, sure you might be playing something but it isn't Overwatch.

Secondly, while I agree that it's best to address people respectfully/constructively, time and experience will show that a good number of people will continue to not give a shit. If someone is dead set on always playing Genji, like the little ponce they are, then they aren't going to care how many times its politely explained, "We didn't need three Genji's, we needed a tank," they're just going to keep playing Genji.

Lastly, and just to address it, while I'm the first person to make fun of someone who takes shit way too seriously, I'm equally annoyed by using "try-hard" as an insult (you didn't say this DemomanTakesSkill, I'm just putting here, this has nothing to do with your argument). God fucking forbid someone put a little effort into understanding and being good at something. Yes it is always about having fun, and taking it too seriously means you're not having fun, but at the same time if we take a view that fun is relative, then you dicking around doing whatever you want may be fun for you, but it isn't fun for the other people on your team who are trying to play the game as intended. Belittling someone or calling them out for trying to be good at a game reeks of insecurity and unnecessary defensiveness.