If you look at what has happened to Barcelona in recent seasons, your statement can be said to be true, that when their star players were no longer at the club, Barcelona began to experience decline and in fact this was also caused by mistakes in managing the club so that Barcelona had a lot of debt, so Barcelona couldn't afford it to regenerate players on time because the key to a club continuing to perform well is regenerating players so that they can produce new stars.
Real Madrid is also no longer with their stars but without financial constraints, they can buy players and now these new players will have the opportunity to become new stars, so it is difficult for Barcelona to compete in the Champions League not entirely because there are no more Neymar and Messi there but also the management's inability manage the club well, but will Barcelona also be a weak team this season in the champions league? It doesn't seem as weak as imagined, just competing to get further will be quite difficult.