Well. Think of it this way:
Fai keeps saying he wants someone to 'take him away' and to 'stay by his side'. NOw, can you imagine Sakura staying with him forever? No. She's gonna marry Syaoran. Sure, they might be all friendly if everything works out and Fai stays in Clow for some reason, but somehow I doubt that's what Fai needs. And really, who else but Kurogane has incentive to stay with Fai forever (especially with the blood thing)?