I've also been wondering about future plans for the XS or replacement models, as I'm sure a lot of people have. Here are my thoughts:

First off, let me say that I love my S90 ES. I've had it since 2006, and I still use it in my bar band.The only thing is, it seems to be getting heavier as I get older. 🙂 But, it has been a work horse. It sounds great, does everything I need it to do, and it has been 100% reliable. I use John Melas software to program the patches to my needs, and I can do everything I want. . . . . Without the software, I may not have been able to set it up the way I wanted, because the keyboard interface is tedious. I customized the patches a lot. I have a separate patch for every song in our set lists, which obviously changes often, So using the software, I'm able to reorder the Master song list as I need to. It works well for me.

The S90 ES product line was in production for five years and then the S90 XS came out; a five year life cycle and then replacement. Technology moves fast (at an increasing rate), so five years seems like a very reasonable time frame for products of this nature.

XS came out in time for the Jan. 2009 NAMM if I recall correctly. According to wikipedia, the ES came out in 2005 and the XS was introduced in 2009.

The XS product line has been in production for over five years, perhaps six if the above dates are correct. This is longer than the ES.

The ES supported the now unfortunately discontinued breath controller and associated plug-in board for wind instrument sound modeling technology. I happen to still use and like the breath controller and the plug-in board, primarily for saxophone patches, and a couple of other wind instruments. I have not heard better technology for this purpose. Although, I've not actually heard the saxophone patches on the XS, I find it hard to believe they would be even as good (let along better) than the ES without a breath controller and the associated modeling technology plug-in board - which was based on far different technology than the other sound synthesis technology.

It's time for a new S series keyboard. Perhaps, Yamaha is rethinking the interface, given the success that Nord is having with their keyboards, which uses a different, kind of analog-ish approach, which people seem to like. I don't think the Nord is as flexible or as powerful from a configuration standpoint as the S series line is, but the Nord does seem to make it easier to get to the power that is available under the hood. The S series interface makes it difficult - without using separate software and a bigger screen, imho. The Nord also has some great acoustic piano, organ and e-piano sounds. But I think Nord's horns and synth section may not be as good.