They both have us by the balls, especially Ozil. He can go pretty much anywhere and improve his wage quite significantly so I see no reason why he's going to stick around. Why would he? We lack ambition and don't compete for the big prizes. The fact that Barca and Bayern are sniffing around doesn't bode well, Wenger had to convince him to stay last summer when Bayern came calling, what convincing can he do now?
Sanchez is a slightly different case, of course he would be courted by many clubs if he's on the market but I think we'll be able to tie him down.
Losing either player will set us back, it's not like we'll replace them like for like if at all.