I must say that the prospect of a front three of Mbappé, Martial and Rashford is extremely exciting, and if it works, will absolutely destroy this league.
A front three of Griezmann, Martial and Rashford would also be massive though tbh.
I also think people shouldn't look at the age too much. I know if we signed him people are concerned we have '3 young strikers' etc, but the game is changing. Next season Martial will be 21 in his third year in England. Rashford similar. That is a fair amount of experience.
Also, many top players are great when young and continue like that, simply because they are that good. R9 never 'developed', he just 'arrived'. Messi was also very special from very young.
Ultimately, it is at this end of the market that the real top money will be spent in future anyway. Once great players establish themselves at big clubs, they do not move anyway, and if hey do, it is usually under circumstances that mean they go for a lower fee. I think the whole 'arms race' element of the market dictates that the next big thing is where most money will be spent.