On paper I completely agree, Max should've given the position back. But in reality I think we both know why he didn't and why I think it was fair game. You even allude to the reason.

The stewards could have given Lando a penalty for that.

Just as they could have punished Lando for it they also could have punished Verstappen for not letting Lando pass. The stewards are wildly inconsistent and if I was a racer I wouldn't put my GP win in the hands of the stewards. Unless I'm clearly in the wrong the most logical course of action is to do what's best for me at that moment and then argue with the stewards later. Another example of stewards being inconsistent is the fact that Verstappen didn't get punished for taking the position back. He absolutely should've gotten punished for it, but he didn't. The stewards play loose with rules so drivers must also play loose with the rules if they want to win.

Imagine if Verstappen had given that position to Norris and then stewards had done nothing to punish Norris. People would have called Verstappen a sucker for giving up the position because why would you willingly give up your position in such a gray area? Verstappen is driving to win and that means he's not going to give up a position just so he could be "in the right". Being in the right doesn't mean you get to win. The winners mindset is that if you can be in the right and win then that's great, but if can't achieve that you'd much rather be in the wrong and win than be in the right and not win. That doesn't apply only to Verstappen, Norris would also be just as fine being in the wrong and winning. Same with a lot of other drivers.