I think its that too much too soon thing. Thats why to be fair to Brendan Rodgers he makes sure although the young players signing there will get very well paid still, a lot of the contract is incentive driven.
Johnson was supposedly on £40k a week without achieving anything and he gets that whether he played or not, despite the injuries you would hope the burning desire to play football everyday would push him on but obviously didnt happen and now he looks a mess.
Hopefully he may see that picture and decide to himself that he has to make the most of his ability and do something now, he is still young enough to do something, rather than look back and think what if in a few years time.