This is turning into the "extraordinary ability in one area assumes extraordinary ability in other areas to make it work" dilemma.
For example, a superhero with super strength can punch through a car and rip out the engine. But, unless they have super-hard skin, their hands will turn into hamburger. So, just because they have the strength to do something doesn't mean they have the other abilities to pull it off.
So, for the monk, if the player wants them to move that fast: fine. I would start adding in damage over time to them for moving so fast.
Feet are not designed to move 100 mph, so the feet would get shredded if bare-footed.
Player will argue they're wearing foot covering (leather boots, sandals, etc). Foot wear is not designed to hold up to running 100mph. And, that speed would generate a lot of friction (both inside the foot wear to the feet, and possibly on other areas of the body as the clothing or leather parts on their body would rub and cause friction burns quite quickly from all the pumping of arms and legs.)
One way to alleviate this would be to assume that the ki-spent Step of the Wind ability literally makes them light as air, so instead of running super fast, they're simply able to fly forward greater distances since their leg muscles are now pushing something light as air instead of normal body weight. (Think of Crouching Tiger Hidden Dragon, where they're using their ki to "fly" through the air and run great distances.) If that's the case, then it could alleviate any idea of friction burns and damage to the body from great speeds. But, if that was the case, I would say they have to pick a direction and stick with it during the turn, because something moving at that great of a speed can't change direction at the drop of a hat - especially if Step of the Wind is having them soar into the air and glide quickly. You can't simply turn in mid-air. So, if you want to allow them to have great speed like that, then the consequence is lack of maneuverability; they pick a direction, head that direction, and can't change it until several rounds later or something.
We're basically having to argue the laws of physics in a fantasy setting. It would be like debating whether the fly spell magically gives people oxygen bubbles over their faces in order to fly high altitudes (to which, if it's argued it did, then magic users would be abusing that to fend off gas traps and stuff).
The way I view this situation is that the players is sounding like a munchkin and min/maxer, focusing on the "can I?" instead of the "should I?"
If they want to do it, and the rules add up to allow it, you have to think of the repercussions of it; there has to be balance in the game. Either the character starts taking some kind of damage from pushing the human body far beyond normal limits, or they suffer severe movement restrictions once they get going.
In regards to the T-Rex: I wonder if the player is thinking their monk has Iron Fist or something, and could use it to punch the T-Rex (like they're running at 10 0mph and just jut an Iron Fist out to hit the T-Rex). Of course, my idea of Iron Fist is from old-school 2nd Edition Oriental Adventures AD&D monks, so maybe your player's monk doesn't have Iron Fist or something.
There's just a lot going on with what you're player is trying to do, and as a GM I would consider it ok or not ok based on context and situation.
The player wants to run 100 mph to go warn some people that enemies are coming: ok, fine, I'd let it fly.
The player wants to run 100 mph to punch a T-Rex to death: no, not fine; you get mushed to goo.
The problem with letting something like this be ok one time is the player gets it in their head that it's ok for anything they can dream up, so they'll start dreaming up abusive ways to use it (e.g. punching a T-Rex). So, it's often better to just either nip it in the bud right now, or come up with some rules to let them know there's a pro/con situation to using it (like damage or movement restrictions).