It's definitely the best decision he made, he will know his worth after he get more trophies with Real Madrid, he will earn more by endorsement.
If you want to get a lot of trophies then Real Madrid is one of the most appropriate clubs for Mbappe because this club has everything. But talking about income really depends on the value of the contract, salary or sponsors who use his services. As before Real Madrid had Ronaldo and Mbappe could be one like him in the next season.
I also see Arsenal as a team that can still be quite strong next season so that some of their closest rivals last season also still have to be wary of Arsenal as their competitor in the race for the Premier League trophy. Likewise in the Champions League because Arsenal could perform better next season and be able to reach a quite crucial stage so that their rivals will still see Arsenal as an opponent that needs to be eliminated early before heading to the quite crucial stages in the Champions League
Arsenal's ability to fight for the Premier League title may still be quite open but to ensure this I am a little doubtful. Especially if we talk about the Champions League which in reality is not their competition because there are many competitors that they are unable to conquer. I am not trying to underestimate Arsenal's ability but this is the highest league and almost many clubs hope to get success there.
For the Champions League I will not include Arsenal as a strong candidate but I still agree if it is related to the Premier League. For two seasons they have always failed to break Man City's dominance and now is the third chance for them. If they fail then I think Arsenal's chance to win the Premier League will disappear again.