Whys is it over the top? I'm not saying he should be getting 20 goals next season as that would be completely too much to expect but making a contribution either from the bench or on the odd start isn't asking too much.
If he gets 6 or 7 goals and adjusts well to the league then that would be fine as far as i'm concerned. If we buy a new striker and sturridge gets hit fitness back then he isn't going to see much game time apart from sub appearances but if he only gets 1 or 2 goals all season and mostly struggles then i wouldn't be at all suprised if the club moves him on.
It's happend too many times before to plenty of players to just dismiss the possibility. Nothing he's done so far in his short career suggests he's ready to be in our first eleven and it can be very hard getting a chance in the team never mind doing well.
I hope he does well but it's hard to have too much confidence in him after a wasted season at Lille and the odd time i have seen him play he was pretty average. Maybe he didn't want to be at Lille anymore and was just going through the motions but if that's the case then he'll have to show much more determination to succeed here.
None of the examples you cite are comparable. This is an investment in a high quality young player with the clear emphasis on realising a return say 2-12 years down the line.
He would have to be catastrophically bad for the club to give up after a single season. You seem to greatly exaggerate the pressure he'll be under in his first season. Some encouraging performances and signs of progress would be perfectly adequate, in all likelihood.