Don't be so sure. If they put in Zen and HBM2 AND Vega, and angle it for next year, that will have been pretty much a generational upgrade, especially on the CPU and RAM sides.
CPU power has been the one thing that has been lacking on consoles since the start of seventh generation, and Zen presumably would put them back in the high tier, even with a lower powered console/notebook variant
We know Vega 10 (aka Greenland) is definitely using HBM2. But that's a huge chip, going to compete with not only GP104 (GTX 1080) but GP100/GP102 as well (meaning Pascal Titan and 1080 Ti) We don't know about Vega 11, it would be either HBM2 *or* GDDR5-X. As with any GPU, i.e. Pascal GP104, GP100, it's one or the other, not both. Too expensive and complicated to have two types of memory controllers on a chip.
Point being, if the intention by AMD to have Polaris 10 and Polaris 11 with GDDR5 and then Vega 10 and Vega 11 with HBM2, then it would seem possible for Xbox Scorpio to have a cut down Vega 11, still with HBM2. However, if only Vega 10 is HBM2 and Vega 11 is just GDDR5-X, there's probably going to be no HBM2 in the new Xbox (although anything is possible if Microsoft wants it).
The disadvantage with Polaris 10 in Scorpio, it would have to be clocked high just to reach 5.5 TFLOPS, as it is in Radeon RX480, and even higher to hit MS's 6 TF target (not counting the CPU).
The advantage with Vega 11, even a cut down version, it could probably be underclocked to hit 6 TF. Thus running cooler, probably lower power consumption, more reliable, etc. and at least the possibility of HBM2, where as there's no chance of it with Polaris 10.