For one I don't see their situations as similar at all. One went back home. The other went north to a despised team and for arsenal fans, "rivals". Fabregas was shocked when van Persie joined our team. Plus they're different people and despite how thick footballers are today, it's not fair to paint them all with the same brush. One player is likely to have more loyalty than another.
Fabregas would consider coming to us if he got desperate for more silverware. However, he's already part of a title-winning team. Although, he's not as integral as he was in his Arsenal days and it looks as if he still feels he has something to prove at Barca. If he's coming back to England, it would probably be for Arsenal. He's not in the same situation as van Persie was and he may choose Arsenal out of respect for Wenger and Arsenal. I know it sounds like hogwash but it's how I view Cesc. Nothing he has done has made me think otherwise.
I'm surprised we're still discussing him coming here because for me, it was simply a way to ease fans concerns about the future with Moyes taking over. As a fan, you're a lot more hopeful for the future when your club is going after top class players. Even if we don't get Fabregas, the idea we tried and were more than interested in this deal will stick in people's minds. Plus I think it diverted some attention away from the Thiago stories.