You didn't say it, but you sure as hell implied it.
go to the quoted post for context of everything i didnt feel like pasting his quote into another quote
having the classes move slow purposefully is not the only solution to this problem
most of the maps usually are one straight path and a few alternative paths that usually cut off from the main path to rejoin it later to allow flanking. on some maps there are choke points where the main path becomes the only route. this is very linear but linearity is also not inherently bad and is probably required in order for the rest of the mechanics to function well.
tracer has the most boring gun in the game and her abilities arent much better. blink is just a general movement tool that lets the player zip around the map and get out of harms way. the distance of blink is large enough that one blink anywhere is usually enough to escape whatever is coming at you and the 3 blink reservoir ensures that anyone who doesnt blink like some kind of dumb ass can get away. as blink is a movement tool it can also be used for chasing and outflanking but due to the slow movement speed tracer has a flowchart of: blink to gain the positional advantage > hopefully kill someone > blink away or press e because blink and e are the only way to get in and out of places quickly. her other e ability whatever its called serves as a get out of damage free card and maybe a forth blink if anyone ever finds the need to dive 3 blinks in. tracers weapons dont really help anything if we are talking skill level as they are point and click raycast automatics that anyone braindead can use.
genji is similar to tracer but more difficult to play in that he does movement stuff a lot except his dash has a bigger cooldown and is more situational because you cant store it, his ranged weapons are semiautomatic slower projectiles, and he has to rely more on his unique wall climbing and double jumping shenanigans to find flanking positions and avoid the big dudes on the front. his only real difficulty thing over tracer is his weapon being harder to hit as he has a bunch of other movement gimmicks and the fastest movespeed in the game to make up for any difficulty with positioning and escaping. you seem to have exclusively picked the movement based glass cannons perhaps assuming that speed = hard to play but i would definitely rank someone like junkrat or lucio or anyone who requires good aim or a lot more situational awareness as up there too. really though the difference between how the skill floor of classes is miniscule while the skill ceiling, while having more variation, is still not much.
i said movement speed was less important than positioning here and you appear to have misinterpreted it. what i mean to say was "and, because of the movement speed, positional advantage is key instead of dodging" so im actually agreeing with you here. team comp also matters but certainly not as much as you are making it out to be and in my experience generally a team can get along just fine as long as there is a tank and a healer, especially in pub games which is what 99% of the people playing this will probably only play, including me and you most likely. im assuming you are talking about skill being like the basics of playing games and yes i agree that skill definitely matters in like every game and that a team that is more skill will defeat a team that is less skilled. the problem i have with overwatch is that the slow movement and general lack of options for each hero diminishes the skill ceiling of the individual which in turn forces people to work together in order to beat the other guys, making the game team based. this formula is undoubtedly a direct and intentional rip from dota and its ilk; those games do the exact same stuff
as for the subjectivity of difficulty as a result of online, yeah you will undoubtedly be stuff on by someone far better than you but my problem with overwatch is that it forbids this kind of stuffting unless its done in a group and yeah playing things together with friends is always fun but a group of friends can play anything and have fun. that one dude whose name i forgot mentioned that tf2 was more individually focused and it is inadvertently because it allows the individual to exert his skills far better with classes like the scout and the soldier but also rewards group play with stuff like the medic and engineer. tf2 is a pretty bad example of this maybe tribes would be better but oh well forget i wrote all of this bullstuff nobody will read anyway.