I don’t think the ability to come up with an implausible scenario where a person could become the thing that seems impossible is the same thing as saying “if you have enough motivation, you can become anything you want”. Both scenarios you described require a lot of things to happen that you are not in control of; space flight changing significantly and colonizing mars aren’t something you can work at.

Also, the height limit isn’t just a made up rule, it is so you can fit in the spaceship.

For the wheelchair basketball player, they can’t just work hard and suddenly the NBA lets people on with bionic legs. That would require a major rule shift, which again is not something you can change through hard work and motivation.

Look, I get why we want to persist the myth that you can accomplish anything though hard work. You CAN accomplish a lot, and probably more than you think, through dedication and hard work, but you definitely can’t if you don’t believe you can. So, we tell ourselves (and others) that you can be anything if you work hard so as to encourage people to try for things they might not try for because they don’t realize they can accomplish the thing if they work hard. It makes some sense to try to delude yourself into thinking you can do anything in order to prevent the situation where you actually could have accomplished the thing you wanted to do, but you didn’t try because you didn’t think you could.

So I get it, but really, if you think it through… there are things out of your control that will factor into whether you can accomplish your goal or not, and you have to be prepared for that, too.