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Cost vs usability.
Ignoring the 'road bike' category for the time being, I'm not dismissing 26" simply because it's been around for yonks and parts can be had for peanuts, even in the middle of nowhere. The argument for 27.5 is also much stronger when you go off road. Roll over all the things! For the road, does it really have as dramatic an effect? I very much doubt it in a blind test with someone who's bike is simply transport.
Building a a bike because you can isn't the same as building a bike you need.
I don't really follow.
If I was to invest effort/time building a road bike and the choice is between 27.5" and 26" why wouldn't I choose 27.5"?