PSG have been silent so far in the transfer window but there is still a long time for the end of it. They have signed only Nuno Mendes so far after his loan deal with PSG has finished. The transfer fee was 38 million euros. He is a good and very young left-back so the deal is a good investment for the future. Maybe PSG are waiting for their manager in next season to be determined before going for some targets. Because Pochettino's targets might not match the targets of a possible new manager.
Maybe they try to close issues with their coach position. After that they will be actively on transfer market. They have a good squad and don't actually need to buy too many more players. 2 or 3 players are enough for PSG to strengthen their competitiveness in Champions League. In addition, they are planning to sell Neymar and would like to use money from this transfer for their transfers in this summer. Lot of things are in waiting mode and Neymar can be a first domino in Paris.
At the moment, his move to Monaco is almost a perfect decision: Monaco plays in the Champions League, Monaco has chances for the top 2 of Ligue 1, which means he will play in a team that will dominate the opponent in most games and he will be able to score a lot of goals. For a career restart, this is the best transition you could think of.
It is good for many reasons.
He can play in Champions League that is dream for Asian players.
He can have a better position and will be able to play more matches than in Liverpool.
He can secure his position in Japan squad for the coming World Cup.
If he plays well in Monaco, his career will be revived in Monaco and he will still be one of successful Japanese players in Europe.