@ Gio
It is not that I don't see your point, and to some extent, I agree very much with what you are saying.
The question is not, for me at least, the skill of Diego, but the level of the team with Aguero compared to Diego. I think it is a wise move because as I understand football, we have a better team after the exchange than before.
I totally agree with, under normal circumstances you should not change key players very often. But nothing in the last 4 years has been normal. I don't think youre Del Neri failure argument is valid either. If he fails, we'll still need a supporting striker. Aguero can play in a 4312, 4321, 4231 and 442. As with DP, I would reckon 4231 suits him the least, but I still think he could play on the sides. If Gio can, aguero can as well. So in terms of failure, this move is not bad.
IMO the chance of Aguero is now. He will leave this summer I think, and then he'll be out of reach unless if he flops. The pros outweigh the cons, and I say go for it, if it at all possible.