How can anyone be 'kept' in a sport when they want to play in another sport, and in a team that they are a fan of and have for a long time wanted to play in?

By making them an offer that they can not refuse ????

Did you never go to your boss and twist their arm for a pay rise or better terms, rather than look for a new job ??

Although not quite the same, this is a similar situation to Gareth Thomas switching from Union to League and although he was an ok League winger, the
guy just didn't have the same impact in League.
Although Sinfield may well have the skill set to do ok in Union, with the best will in the world, it will take a couple of seasons to get up to speed, by which time
he will be 37, some way past his prime.
It's hard to see what he can achieve, other than the pleasure of trying another sport, which for a top sportsman is not much of a challenge.

Another sport that he's followed for years and always wanted to play, for a team that he supports. Obviously its not all about money with him, which has to be admired.