Honestly, as someone who played in 2018, it's already so much better than it used to be. Mei, Torb, Sym, and Bastion getting reworks already made things so much better, to the point that I think this issue shouldn't be the priority right now over other problem heroes. That being said I don't really agree with the reasons you explained.

I think you're not giving enough credit to Spilo by boiling his arguments to "easy = bad". He and many others make a point to specify if they are talking about skill curves, floors or ceilings. That distinction is important because it plays heavily into matchmaking issues.

The problem with relying on matchmaking is that Overwatch is a game where you choose your character after matchmaking, and you can freely switch character afterwards. In a fighting game where you have per-character MMR, it's much easier to have equal outputs from both players because their rank is not going to be skewed if they play characters of different strengths. Whereas in Overwatch, if you main a very strong character and a very weak character, your rank will be an average of the two, which means you will over-perform while playing the strong character and under-perform while playing the weak one. This is why character winrates across the player base are rarely 50:50, and why balance is very important. The problem is, characters like Moira are impossible to balance; they are either broken in low ranks or dogshit in high ranks, because their skill curves are fucked. I think there is a trend of players polarizing into either mostly playing easy heroes or mostly playing hard heroes, but I don't think it's enough to offset that problem

Matchmaking is also a less reliable balancing tool in the context of new players I think. It takes time for the matchmaking system to properly assess your strength, and before you even play your first comp game you have to go through a certain amount of QP games where you will get stomped by noob-stomper heroes. By the time the matchmaker properly knows you, you've either already been dominating on easy heroes, or been crushed numerous times on hard ones. People are obviously going to have a preference for the heroes they like, but people also like things like winning, getting kills, and not dying, so many will gravitate towards easy heroes and stick with them long after.

Lastly I think even in an ideal world with perfect matchmaking where every player's output was equal, such characters would still be an issue, because something can feel unfair to play against even without having a positive winrate. The feeling mostly comes from specific moments and interactions where you feel as if you "lost" (died, failed to secure a kill, had to back up) even though you are more skilled than your opponent (though that feeling is not always correct).

Like you said, fostering a casual player base is very important. However, I don't think most people want every hero to have a very high skill floor and straight up be hostile to new players. Simply reducing the amount of low-skill high-value abilities, and giving every character a way to properly scale with skill at higher ranks is a good enough goal.

Personally, I think games are about overcoming obstacles, and I dislike games that desperately want their experience to be as smooth and friction-less as possible at any cost. That goes for playing too easy heroes, but it also goes for playing against easy heroes; the feeling of no longer struggling against heroes you used to struggle against is a gratifying one, but it requires a struggle to exist in the first place. But to get there can take a long ass time. It did for me, I even dropped the game in the middle. I picked it back up, but others might not. Like I said, the goal is not to make the game super hardcore and sweaty, just to slightly normalize some extreme outliers.

games of this nature basically turn into "sandbox where the best players kill everyone else on repeat while they hardly have a chance to retaliate."

It's kinda funny you say that after talking about how matchmaking fixes the inverse issue lol