In an ideal world, we'd sell Nasri for 25M and then reinvest that fully to buy someone of similar quality (Mata, Sneijder, Schweinsteiger). Repeat for Fabregas.
This isn't an ideal world, we already have roughly 26M profit made from this summer burning a hole in Wenger's pocket, even if Nasri goes for just 20M, can anyone see Wenger splashing out 46M on top quality players?
Unlikely. We're more than likely to start the season with what we have or buy lower than acceptable quality (someone like Scott Dann), for which we don't need Nasri's fee at all.
Sure, it's silly to make a 25M loss in business terms, but how does the club netting 25M benefit the fans? It does not benefit us in any way at all.
If ticket prices came down, there would be some kind of excuse but they've risen by 6.5% this year, a move that nets the club only 4.5M and has pissed off 60,000 people. Despite the club probably banking at least 30M in profit this window, very little of that will be spent on anything that boosts the fans.
Nasri's still our best attacking midfielder and he is a professional. You can't slaughter his attitude for dropping off in the second half of the season and then ignore fan darlings like Wilshere, Walcott, Sagna, Djourou etc who did the same.
In conclusion, a quality player on the pitch (even for just a year) > the board making 25M profit which we'll never see.