I know it's been done to death on this forum, but there's relatively recent precedent for this sort of race between a popular R governor and Generic D in the form of HI-Sen 2012. Linda Lingle was probably the most popular governor in the state's history, had tons of money, and was polling within striking distance of the Democrats in summer 2012 (i.e. not two full years out) and in the end she still lost by 25 points to Mazie Hirono, even though she had beaten Hirono in 2002 to become governor in the first place. I realize that Hawaii and Vermont are different in many ways, but they are similarly partisan and intractable on the national stage.
I guess what I'm saying is that a lot of times R governors can get away with great approvals in hyper D states (or vice versa) because Ds in those states have a lot less to lose. After all, Hawaii or Massachusetts or Vermont residents will never ever have to worry about losses on the state level on important issues like abortion, so R governors in those states have the built in crossover appeal of being forced into moderation along with near unanimous approval from Rs, who are just happy they're finally being represented. As soon as those governors head to D.C., though, they have to answer tough questions about those same hot button issues, which are suddenly relevant, and assuaging those concerns is a tall order when at the end of the day being elected as a Republican means you're beholden to Republican leadership. I could be wrong, I suppose, but my instinct is that Scott knows this and won't run, and if he does run I have a hard time seeing how he doesn't go the way of Lingle. After all, Scott would probably have to outperform Milne by at least 15 points in order to *maybe* eke out a victory.
Linda Lingle had quite mediocre approval ratings at the end of her term in spite of her landslide victories. Her race was much more of an uphill battle than a hypothetical race for Scott would presumptively be.
https://www.staradvertiser.com/2010/10/25/hawaii-news/most-surveyed-voters-dislike-lingles-performance/