In an ideal world, I'd defo agree with you, because we'd surely be entitled to 30+mill upwards at a tribunal when his contract expires. BUT, there's talk of him engaging a very very rarely used loophole called the 'webster ruling', which means he can hand in his notice a few weeks before the end of next season, and buy out his contract for a couple of million and leave on a free. And there will be no compensation.
From what I've read this webster ruling has only been used once by the initial claimant andy webster, but in theory sterling could try it.
He'd leave himself open to being sued and that after expensive court proceedings and no doubt appeals - a lengthy process. It would also almost certainly go to the Dispute Resolution Chamber of FIFA and/or CAS who decide on the market rate for the player. I also read that most top level player have it written into the contract that if they unilaterally terminate their contract, the market rate of the player will be classed as the harm suffered by the club. I don't know if Sterling has this clause but I suspect it's standard now.