The era of paying players for past success is over. Kershaw has already gone down as one of the best pitchers in history. He's a guaranteed first ballot HOFer. 10 years ago he would have been given 10/300 to be a shell of his HOF self and to be replacement level for the majority of the contract. That ship has sailed, teams have finally learned. There are waaaaaaaaaay too many red flags on him and he has to know it.
With that said, he would probably be able to get an extra $40-50M but no more than that. It may be worth the gamble to ride out the remainder of his deal and get more money in the end if he isn't a #4 type by then.