1. You (& others) think he was the best player. Doesn't necessarily make it true. I, for example, don't think he was. And that also does not change the fact that Diego was considered to be a flop by a big part of the (Italian) audience. Like I said, regardless if it was true or not, it's what happened. Fan favourite? Sure, just like Grosso was a fan favourite for some. Or Iaquinta. Or ...
2. First of all, it's not a 10 million loss. Second, we had (& have) very few players who would bring in real money from a transfer. Diego was one of them, maybe that's why he was sacrificed.
3. I haven't seen every game in preseason & I missed one of the EL qualifiers, so I obviously don't have the complete picture. What I did see however was more of the same: Diego trying hard, achieving some success from time to time, dropping back deep and ultimately hurting our attack (no balance whatsoever) as much as helping it. For me, he's no support striker. I've been saying that for a while now, and I don't think I'll ever change my opinion.
4. Would indeed be a weird reason, but who knows.
Look, like I said, I don't know the reasoning behind the deal & I don't claim to either. Alen asked what other reasons there might be for his departure, besides the DP thing, and I gave him a few possibilities.
Now you may not agree with them (neither do I for some), but there's no denying that these are indeed valid possibilities.