Again, I'll repeat, you are jumping to conclusions saying he is declining when the whole team is underperforming, there is clearly something going on at the club.
That is not even me saying we should sign him, just pointing out that CLEAR SIGNS OF DECLINE is not something that is fact
As for his age 28 is not old for a CB
Of course the Napoli's failure this season is not only on him, but you cant deny he hasnt been the same as last year. We should had learned by now that its a mistake to pay top fees for players that are showing signs of decline, we experienced it with Schweinsteiger and Alexis.
He will be 29 in june, so he has 3-4 years left at top level and then after he's 31-32 you'll have to sell him for peanuts. So basically he has no return value, for that kind of fee I think we should either get a totally proven player in his prime or a younger player that still has 8-10 years of football.
I think for less than that fee we could get way better players like Gimenez from Atletico or Upamecano from Leipzig who are in a better moment and are way younger so they will only get better.
Also take into account that serie A level is not the same as the premier league, look at Smalling looking worldclass while here he was severely criticized.