Not that I have anything new to add - but I called this when he was signed.
The reality is he signed 6 months out from a RWC, with that tournament being his big carrot.
People are saying the RFU mismanaged the situation - and they are right in that they led him to believe he would be ready to play in a RWC after 6 months - but I doubt if he didn't make the RWC squad he'd still be sticking around either. As it was, it seems he hasn't especially enjoyed his time in union. His big 'carrot' of a RWC ended up in England's worst ever tournament at home, uncertainty whether he was a forward or a back, him losing money to poor market speculating, and pretty constant media circus surrounding whether he should/shouldn't be in the squad. If he wasn't selected for England's squad he'd likely be in a similar situation of feelling under-valued.
In reality I think he wanted to play in a RWC - not necessarily be a rugby union player.
Sonny Bill Williams is probably a bit the same. As much as his popularity is on the up - he has definetely picked his times to be a rugby union player in New Zealand. 2010-2012 - enough time to make a bid at rugby union in NZ to make the 2011 RWC squad, while also giving him time to play in Japan and still make the NRL season. 20114-2016 - enough time to be included on the EOYT and make another bid for the RWC, and 2016 allows him access to the Olympics. He may stick around for 2017 with a Lions tour to go.
The point I'm making (and not taking anything away from him) - is that his career ambitions seem to be very 'reward oriented' - rather than based on developing with one team and being the best he could possibly be at that one thing.
For Sam Burgess, he is a legend in rugby league. Whatever people think of his 'potential' in union - I think they are naieve to think a return to being the pinnacle wouldn't have always been more alluring. Nothing about this at all has surprised me.