I don't think the situation is as dire as you say it is. We really don't have a massive problem with the ageing squad like you said. Demiral, De Ligt, Romero, Rabiot, Arthur, Kulusevski, Pellegrini and Bentancur form a very youthful bunch, paired with Dybala, Ramsey, Costa, Berna and Sandro who are in their prime (in terms of age, not form...). Bar Ronaldo, Bonucci and Cuadrado, our starting XI is actually very youthful, and those three are all playing better than their younger counter-parts.
High salaries is a problem, but getting rid of some dead weight like Higuain, Khedira, Matuidi, Berna, Rugani, De Shitlio and maybe Ramsey would ease that problem very much.
We have a lot of quality players and they are all suited for Pirlos preferred 4-3-3 which the players are already familiar with. It could be that with just a couple of player additions, a change of attitude and a new tactical approach, we could start to get the best out of our existing players. I don't think we need a massive player overhaul; add in a regista like Tonali, sign some depth at full-back and maybe get a new striker to replace Higgy and we're good to go.