Think salary cap is definitely a big part of it - I imagine salaries for younger players/academy grads has gone up, which means less money for the top guys
I feel like the idea of challenging yourself abroad is also becoming more attractive - if you can make more money and experience a different country/culture/league, especially immediately after a RWC so you'd have time to come back and challenge for the next RWC, why not give it a go?
Ribbans is a bit different to, say, the Exe lot leaving, though - from South Africa and played professionally there before moving the England, so it's not like this is Saints losing a dyed in the wool home grown kinda player