Of course neymar could struggle like di maria did, but those instances are rare where a proven world class player struggles after moving to the prem. Its really not that hard to believe that an unproven 19 year old kid on the back of 1 breakthrough season is more likely to struggle than a player who's been performing at the highest level in europe and the World Cup consistently. The most likely outcome is certainly one of Mbappe struggling to come to terms with the increased intensity in competition and going through the 'second season' syndrome that all young players go through where they are paid more attention to by opponents. Neymar has already gone through and past that stage in his career.
Someone made the comparison as to how Rashford might score goals in France.
Turn that round & look at how many times a simple punt upfield here, or a defender falling over, gives Rashford a chance to use his pace, which he does, then 8 times out of ten, hits the keeper, falls on his arse or smacks it into row Z. Then look at Mbappe's finishing by comparison & also his positioning & his foot movement & he's faster than Rashford.
In the final 3rd, he is a nightmare to play against, at any level. He is already doing what Neymar can do, in that area.
The difference is that Neymar can drop deep & contribute more, at this stage, but he has never done that outside his comfort zones, where he gets a free kick if someone farts next to him.
It's splitting hairs to suggest he would automatically be better, simply because he's older. Mbappe, if it was ever possible to sign him, would not be a gamble.