To be fair I don't think we'll have a "best player in the world" after Messi and Ronaldo will retire. We'll have around 5-6 players at the top tier and they will alter according to form and trophies.
Rashford will not be one of the elite pack. He is a cracking player but not one of the very best.
Greenwood has a chance if he can sort himself out.
Agreed with most of what you said.
But imo Rashford can be a world class winger.
I still remember his performance against Spurs last season, and against City, both before the break. That moment I believed he's on the verge of stepping up to a new level, a world class level. Because he started to deliver amazing things on a pretty regular basis. Then he got injured and couldn't find that form again.
I really hope he can regain that form back. He's been hitting a pretty good form for the last two matches. Not to that level yet. But I do believe he can. Not his pace or tricks or anything, imo his biggest strength is his mentality. He wasn't born with a great deal of natural talent but he has the mentality to become a world class player.