I think the simple answer to it is this.
Nobody knows when Yamaha will produce new models, but as the PSR SX-700 is a pretty new addition, I can't see an update coming along any time soon.
You will always have a scenario where newer models will have more features than the ones they replace . That's just market forces at work, but you may also get a newer keyboard that doesn't fit your bill at all.
There seems to be more of a lean towards combining arrangers with synths, and that's something that really wouldn't interest me at all. For the sort of music I play the SX series are, I truly believe to be the very very best arranger keyboards on the market at the present time. And you can add to, and customise them so much already.
But the ultimate choice is always yours. As I always say....Spend Once Spend wisely.
Keith