I'd say its less about the cooling. I am sure they will design a very good cooling solution, probably even better than ours since they got a massive R&D budget. I remember someone somewhere said that the R&D budget of the Surface Book hinge alone is roughly as large as the entire Eve V.
Its more of the perception. It is no doubt that the first-gen Core M was underperforming, and the stigma is still among many of us. The moment people hear Core M, they got an image of a sluggish but expensive laptop, like the MacBook 12". I know a lot of people who got turned off the moment they heard the V has Core M.
Oh, and imagine what the news sites will say, "Surface Pro 5 released with upgraded screen, downgraded to Core M"
"Surface Pro 5 followed Apple's lead in using Core M"
"Turns out the MacBook was right all along, Microsoft followed the use of Y-series CPU"