It's still a career at the end of the day. Just because they're footballers, it doesn't mean that they can't take advantage of a situation that allows them to make themselves even more financially comfortable, we'd all do it if we had the chance. This is hardly a new thing, either; Pele, Batistuta, the De Boers, Guardiola, Enrique, Raul and then more recently the likes of Asamoah Gyan, and then there was the El Dorado spell of Colombian football, too.
Oscar and Witsel, they weren't going to move to giant clubs who were going to win the Champions League, so why not move to China? at 30+, neither were Martinez, who'd ultimately failed at Madrid, Lavezzi, Mbia or Pelle? Sure, you could plod along at a mid tier side at the top level, maybe play for 2 more years, or you could head out to China as a superstar and play another 4, whilst making big money.
The CSL isn't a new revelation, they've been doing this for a number of years now, usually with young South American talent, and now they've turned their sights to high profile players and it's shaping up to be quite an exciting league, for those willing to take the time to watch it.