No one thought that Chelsea, which we have known as one of the giant Premier League teams, would turn into a team that anyone has a chance of beating. I think their decline started after Abramovich left Chelsea. Todd Boehly is actually someone who is not stingy and he is willing to spend a lot of money on players, but I think there are differences in management, staff selection, and player transfer policies that made Chelsea become a failure.
Abramovich is not a patient boss and he can quickly sack a coach if he feels the team does not have good form and some conflicts between coach and players is harmful for the club. Boehly is not different as he already sacked some coaches so far and he is ready to spend big transfer budget for the club. The big difference between two bosses is Abramovich is more wisely and sensitively with his decisions, from player to coach and perhaps he received good support from the club Sport director. Boehly should be responsible for his bad decisions for Chelsea so far, spent a lot of money for player transfers but can not help the club to improve in quality.
Chelsea can fall into long crisis like Arsenal and Manchester United and will need many years to return to top four.