Chelsea is a rather young team in the Premier League. The average age of their team is ranked 6th in the league with Arsenal being first (24.6 years) and Fulham being the last (27.9 years). Rejuvenation isn't the problem. They finally need a coach and players who are absolutely willing to put in the effort to be successful. It was never seen in soccer that a club spent so much money on transfers and couldn't even qualify for the European tournaments, not even the Conference League. Those sunk costs are higher than everything else we have ever seen.
The average age of the old players shouldn't be a problem if the players are still able to do hard training, so that when the performance on the field is good and reliable, experience will be an important key for the old players. So it's better to save money to buy players who really need it than just for rejuvenation.