Considering there was already so much confusion about the ARM-based Surface vs the x86-based Surface Pro line, I don't think they want to introduce additional confusion by also having the "Surface" name now apply to phones and restart that headache.
Perhaps average consumers don't care about processor architecture, but they do care about what programs will run. Surface RT was "that tablet that kinda has a desktop but can only run Office" versus "that tablet that can run any Windows program from 1995 onwards".
What they need is a surface phone. One phone, three variants.
Surface Phone - Low-Mid Range
Surface Pro Phone - High end w/continuum
Surface Pro+ Phone - High end Phablet w/continuum
That would match their naming for the tablet and cause little confusion about what they are in the minds of consumers.