Look, I love Gigi, I was a goalkeeper too in my young(er) days and he was one of my idols (along with Peruzzi, I loved him), but the fact is that we can not compete financially with the worlds biggest clubs (Barca, Real, Man Utd, Chelsea, etc, maybe even Inter).
We cant afford to lose out on returning the money spent on players as we dont have enough new money to invest every season. It's all about getting a good player, playing him for a few years and selling him just before his value starts dropping significantly.
Gigi will have to be replaced sooner or later, the only question is what we can get from his sale to replace him, 40 mil € now or 0 later.
Gigi has been injured a lot this season, and IMO our performances werent that closely related to his form, that is we werent significantly worse with Manninger instead of him.
We need a small revolution this summer if we want to get back to winning ways, and imo it's the right time to replace Gigi and use the money from his sale to get a replacement and spend it on new players.
Would we be much worse if we had another goalkeeper combined with making the rest of the squad stronger? It can be argued that Gigi is one of the best but perhaps not clearly the best goalkeeper in the world anymore, and their form fluctuates, as with any other player. Julio Cesar was for a while in better form than Gigi, it's all normal.
IMO, we would not be weaker, we could be even stronger if we rejuvenated the squad. While there may not be a goalkeeper quite as good as Gigi, a slightly worse, but younger goalkeeper, coupeled with adding new players on key positions, could mean that the team as a whole would be better.
IMO it's time to sell him if we get a decent 25-30 mil € offer, or maybe we could get even more from Man City or someone else.