Since Intel has abandoned the ATOM SoC designed for Tablets, the X3, X5 and X7 Tablets are all very similar in SPECs and there isn't an upgrade path. The only successor path is using the Skylake (or future CabyLake) M3-M7 (which the OPP SP4 uses). Currently the Core M Tablets are all trying to compete with the SP4 with 12" screens.
I'm hoping we will see an ARM based Surface Phablet (6.5 - 8") with Ntrig Support running Windows 10 AU with Continuum Support.
Well, I agree and I am bemoaning the fact!!!