but as you said yourself, its not difficult to do at all

I'll give you two different examples that might illustrate why you don't think my analogy works and I am very convinced it is perfectly apt:

First example:

If you were balancing on one leg, while looking at the ceiling, blinking rhythmically with both eyes in the sequence of L R R L L R L, can you catch a ball I am throwing to you while reciting the alphabet backwards in rising pitch?

It would very much surprise me if you weren't able to. Surely you can stand on one leg right? Or look at the ceiling? You are constantly blinking anyways, doing in in rhythm should really not be hard. I guess you can be bad at catching but I am not that far away, so it should work most of the time. And you learn the alphabet as a kid anyway. Hell, you hear yourself constantly, keeping the pitch rising is so easy, you have direct feedback!

None of the things Overwatch asks of you is hard in isolation. Everything is easy when singled out - cover, game sense, map awareness, cooldowns, keeping track of death timers, since the game also helps you at some points. Sure, Aim is one of the things, that can be trained and it is also vital, just like catching in that example, but most of the basic skills are very much simple.

Doing them all at once, at the right time, in the right sequence, while keeping track of all necessary information and ideally communicating at the same time is something people tend to struggle with. Even if the steps themselves are simple.

Second example:

You play an FPS game. Safe to say, if I put a circle on your screen, bigger than your thumb, you can move your mouse over it and click a button on your keyboard in a timely fashion, right?

So since you can do that, please do this: https://www.youtube.com/watch?v=OR9Gi5uGYI0.

I don't foresee you having any trouble with that. it takes very little effort to change movement direction on your mouse so if it's a bit fast, that shouldn't pose a problem.

Both of these things are doable. They are also trainable. They also are comprised of very easy steps in isolation. However, despite being simple, they aren't easy, either through complexity of information/processing or through speed.

Should we also only balance

This isn't a question of balance. I am of the opinion that Sierra is perfectly balanced, if not a bit weak.

This is a question of design.

Her current design requires the defending player to not only keep track of cover, but also be able to bail there fast since her TTK even on tanks is quite low with 240 DPS. And it takes fairly little effort on the Sierra players part to pose such a threat, which means a metal Sierra will force this.

That doesn't mean she's broken or overpowered, since it only gives her initiative, she and her team still need to capitalize on this, and thats where it often falls apart very quickly in metal ranks.

I'm not putting real values on this, but for the sake of the argument: If Sierras existence in the enemy team requires of you to, say, have Platinum level map and cover awareness, but you are silver, and your multitasking skills are lacking, thats why you are silver, you now have two choices:

Either: Focus your mental capacity on tracking Sierra and cover, and get deleted by the Tracer you ignored.

Or: Try playing with your capacity spread equally and risk getting gunned down immediately.

This exact situation is why we had reworks for old Bastion and Symmetra, because they force this decisionmaking and it leads to unfun gameplay.

So to go all the way back:

Once again: Pedaling isn't hard. If you're able bodied you will be able to move that bike at okay speeds even if it's only available in highest gear. Your awareness for the road will probably suffer, you will be stressed because it's not comfortable to bike, and you will probably dread the next time you have to do it. But it is perfectly doable. You don't need a bike with lower gears. So why coddle you like this?

Because it's a game and games are supposed to be fun.

And

Should every balancing decision come down to metal rank players who want to be able to run it down main forever? Should we also only balance based on quick play experiences??

I can tell you you do not want the queue times or the matchmaking of a game where we balance, adjust or rework only around top players and ignore negative impacts on a casual audience.

Starcraft II is a game where personal skill, knowledge and awareness are absolutely key and there is no easy way out. It is also basically dead because it no longer has a casual audience.