Really? what is Higuain better than him at?
Scoring,Strength,holding the ball under pressure, making great passes when pressured, off the ball movement... Just pick an attribute that is different from age and he is better... It doesnt mean Mbappe isnt good or isnt going to be better - He is still very very young and has a lot to improve
Would I want him here?! ofcourse but Higuain is currently the better player! no doubt about it!
Dybala also have a great future and amazing talent but Tevez tenure here showed that 2015 Tevez>Dybala (current one not sure about 2 years from now though...)
Ousame Dembele is probably the next big thing at LW but is he currently better than Robben?! I doubt it..
Anyway best for the future is definitely a trio of what you said as Mbappe is going to be huge while Sanchez is already huge and Dybala is growing