I just picked up a used s70xs and it's just so awesome. I highly recommend it. Only negative is that it's a little heavy. I can boost the highs on the piano sounds to make it supercede the venerable CP-300. It's got another one: a clear but less pointed S6 Natural Grand; and the original Full Concert Grand from the Motifs too, which I still really like, by itself and also in a Performance stacked with the new S6. it doesn't have the ES's S700 Piano, but I haven't fallen for that one anyway, so I don't miss it. The EP's are great. The Pads. Organs. Brass. Clients love the Strygs...Everything about it. Then add the thoughtful layout, and it's a clear winner.
If Yamaha is coming out with anything to 'replace' it, who knows if it's gonna even be universally agreed upon as an improvement? I never 'got' the CP-1, e.g.
I'd go with the combination of excellence and price discount for not being a new release, and pick one up when you're ready.

The Series-S (S90/70 XS) replacement question is a popular question that has been discussed for a few years now. As both a S90XS and S70Xs owner I myself hope the product is supported for many more years and doesn't fall onto the history lists anytime soon. The S90/70 XS is a board that does many things well and has a great feeling set of weighted action keys. Recently I picked up a used Motif XS6 on EBAY. I started using the Motif XS6 to build drum arpeggio for some old classic songs I cover. I copy those arpeggio over to the USR arpeggio memory of my Series-S boards. Yes the S90/70 XS boards are weighty but that solid metal case construction is going to help make them last decades. Also once on the stand that weight keeps the board solidly still. Nothing worst than a board that starts bouncing off the stand during a four to the floor rock song.:) I've also used my S90XS board to build original songs recording audio and MIDI into my Sonar based DAW. My current thing is building two man band Performances where I do vocal along with left hand doing bass and right hand doing keys on the S70XS, the S70XS does drum and some rhythm arpeggio with the guitarist doing acoustic electric guitar and vocal. I use a MIDI foot pedal to turn arpeggios on and off during songs. There are very few boards on the market that one can do this so easily with. I hope Yamaha keeps the Series-S as is for a long time. If a new board does come out, hopefully it will be very backward compatible and have an XS mode on it.

God I hope Yamaha doesn't obsolete the S90ES. I bought a used S90ES wanting to integrate it with Cubase 8 Pro. The S90ES is still going for around $1,400.00 used. So, for Yamaha to stop supporting it would be a huge turnoff for anything Yamaha in the future.