I think he should leave for a better side with guarantee of playing first team football, my reason is that most of the top players in Monaco are all leaving, next season won't be so good to him at Monaco as his team mates are already leaving, and it would take sometime for their replacements to settle in, he is most likely to struggle to adapt with the new replacements. Silva has left to Manchester City, Bakayoko deal with Chelsea is almost done, several other players in the team are also linked to various teams in Europe. So it will be a great test for Mbappe if he remain at Monaco and there might be a risk of devaluation if things didn't work out well later on. If the absence of his team mates affects his performance in the coming season, then his worth will reduce automatically which is a risk in side of Monaco.