You say that Wilder will find it hard to gain mass, but, being a heavyweight, will this matter much? It's not like he will have to gain weight to move up in weight. Now, I understand if he could build his legs up then this will help his punch resistance, but if his chin is OK for now, and he is able to avoid opponents shots due to being quicker than them (for whatever reason), will it matter much? I can see how eventually it may become a problem one day, but with what he has now, it seems to be working well for him and I still think he can achieve a lot. Will he become an all time great? Maybe not, but he can still have a successful career.
Keeping someone smaller than you at distance for 12 rounds isn't as simple as it seems, but I do get what you are saying. However, in todays heavyweight scene, a big guy who can do that can have an incredibly successful career. Just look at the Klitchko's. Because most heaveyweights these days are between 6'0"-6'4" and are generally plodding and slow of foot, a tall fighter who can use his height advantage and control distance is pretty much unbeatable. These smaller, slower HW's only really stand a punchers chance of winning. As for his power, it is clearly good. Will have to see him against more live opponents to see if it is devastating power, but I would say that he certainly has enough to hurt anyone.
This isn't bodybuilding. Wilder's legs are clearly pretty damn strong. Would he be punching this hard with weak legs?
No, it's genetics. Strength and mass follows quite well. A weightlifter has small legs compared to a bodybuilder but large enough to squat heavy weights. And 18' inch thighs is not large enough... The key difference is the ability to take a punch. Ali had between 25-27' inch thighs during his career. Couldn't punch as hard as Wilder but his chin could take the punishment to become a champion. Even champion Wladimir Klitschko gets criticized for his suspect chin - in reality he has pretty weak legs himself in just 22' inch thighs. I favor all tall boxers with some speed over Wilder. Fury, Fury, Joshua, Parker... even an old Tony Thompson etc.
I'd say Wilder has pretty functional strength. He doesn't punch this hard with weak legs. He has skinny legs. Doesn't seem to affect his power. Ask the guys who get hit by him.
The difference is when he has to take a punch. About his power: he is fast and rangy, yes and that's why he can connect with his wild (and risky) haymakers - against smaller, slower and not as skilled opponents. When he do meet someone and has to box with straight punches - his power will not look as good anymore.