I honestly think that Romney, if he ran, could do pretty well in a primary. While all the other candidates tried to out-Trump each other, he could carve out a niche by dominating among the significant minority of GOP primary voters who are not that into Trump, standing out against a fractured field.
Significant miniority? What significant miniority? All 10% of them?
Even 10% is enough to gain some traction if you have 15 candidates splitting up the pro-Trump vote. And anyway, 10% is just the number who are positively anti-Trump, there are others who approved of him because of the tax cuts, judges etc., but would prefer a more establishment fiscal conservative. The donor class would surely coalesce around Romney in a way they never did around a single candidate in 2016.