I would think Yamaha could do this if they wanted to, but it does not seem to be the case that they feel that they need this in their lineup. Back in the early 1990's, they had a logical progression of PSR-100, 200, 300, 400, 500, and 600. And then I think they went to something like the PSR-1000 and perhaps higher-end models (I'm really not sure about the higher end models of that series), and the PSR-500 and 600 would've probably been on the lines of what you are proposing today. They came out with newer generations, and still had a similar progression in their line-up, though sometimes, the model numbers got a little wonky (for example, as far as I can tell, when there was the PSR-510, there did not appear to be a 610, but there were PSR-1700 and PSR-2700 models around that time which would've more or less taken those "slots".
And a while back, when the "S" series was a newer concept, such as with the original PSR-S700 and PSR-S900, there was, indeed, a PSR-S500 -- something that can cause confusion with the original PSR-500 (no "S") as those are two distinct keyboards from different eras. But for whatever reason, the "500" series was discontinued years ago and was never really re-instated. For the main arranger/home keyboards, it goes from the PSR-E400 series (currently the PSR-E473) to the PSR-SX600.
I always felt that this put a gaping hole big enough to drive a truck through in the middle of their line-up. You're going from a $370 keyboard to one that is currently about $1100 (US dollars). Adding the PSR-EW series, which is essentially like the E but with 76 keys and a few additional features, helps a little, as the PSR-EW425 (the current EW version of the E473) is a little under $500. But that still leaves a pretty big gap in the line-up.
I guess Yamaha feels that they have keyboards like the DGX-670 (currently about $850) that help fill in the gap, but of course, that's a little different since it has 88 weighted keys. It would be cool to see either a PSR-E500 series or a PSR-SX500, but it doesn't seem to be a high priority for Yamaha at this time.