I will avoid the fact that a 90% winrate in a format like you describe (heads up sit and go) is not even remotely possible vs anyone who knows the basic rules. In heads up sit and goes 2-5% roi is generally considered great at any non micro level. Poker (especially online) is more of a volume game. However, roi is very flawed which I will get to. Generally the most profitable players are ones who crank out a small winrate over large amounts of volume.
Here is the problem though, we really are not concerned with winning a single game in a format like this. Our top concern is making money. In a winner take all heads up tournament chipEV == $EV so we can just call it EV in this format and not worry about ICM. If we are playing for a living we want to make the most amount of money, so each session we want to maximize our EV (win the most amount of chips). Most internet players worry about their winrate in BB/100 or EV adjusted $/hr.
When we get into a situation which we are a 1% favorite for full stacks vs a player we have a large edge on, we should still take it, its money in our pocket long term. If we lose we pray the opponent accepts a rematch. This is especially true for heads up games which usually only last a few minutes and its possible to play 100-200 games per day. A 3% winrate at $100 stakes with this volume becomes a significant amount of money even before factoring in site bonuses (likely a bit over $1/game) etc.
Not related to your comment, but doesn't deserve another post is that poker really is a great game and its unfortunate how its perceived by so many. Ive been playing for 3 years professionally and coached on a popular training site, yet after transitioning to programming (With a cs degree) I get laughed at by recruiters and seen as a degenerate.