PSR-S670 seems like a successful product. I speculate that Yamaha would've never made such a powerful keyboard in this price range if Korg hadn't pressured them with products like PA300.
Previous keyboards like PSR-S650 and PSR-S550 were much less usable in my opinion, while KORG was offering better polyphony and a touch screen.
Yamaha released years ago PSR-550, which was an entry-level keyboard with a very powerful user interface, and they kept producing it for a quite a long time. Now they keep PSR-S670 on the market longer than other products and it sells very well.
Yamaha still protects their more expensive keyboards, so they made PSR-S670 without buttons at the screen making the navigation less convenient, but there's no obvious barrier to use it to its fullest, unlike it was with previous models, and it does seem like this time Yamaha did steal some popularity from their own TOTL arrangers in order to fight offers from other companies.