You are right. Chelsea has done many useless purchasing. They bring back Lukaku, but it was useless. They signed Timo Werner, then they sold him because he failed to give a good contribution to Chelsea this season. They will sign Bellingham and this player may be useless for Chelsea squad. So, Chelsea has a habit to waste money.

I think the problem is on Chelsea management, not on the players. 

Yes. Bellingham will be a perfect player to replace Modric. Bellingham is a midfielder with big talent and he is very young (19 years old). He will have a long career in Real Madrid.