Depends on who/what/when/where. In another couple years that post could be meaningless if someone breaks the transfer record. Pogba is the best young central midfielder in the world and those are hard to come by. I'd rather keep him until we reach his maturity, sort of like a 5-year bond.
I can appreciate that. But I think history has proven that taking the money on offer and re-investing it yields positive results. Look at Zidane, 75 million euro is insane money now, let alone for back then. We took that money and bought Buffon, Thuram and Nedved. More recently, look at Napoli. They received just under 100 million euro from PSG for Cavani and Lavezzi, and with that they bought in Higuain, Callejon and Mertens and as much as I dislike Napoli, you have to admit that they're a better side for it. 100 million euro can benefit us a lot. Buffon and Pirlo aren't getting any younger and will need to be replaced in the near future, I wouldn't mind some extra defensive cover and maybe another midfielder and even another attacking option. I think we could fill a lot of holes with 100 million euro, and to forego that for one player, no matter how good he is or will be, is a mistake.