You're assuming MAXON only started the core rewrite after R16 or thereabouts. In fact I think they've been working on the new core for the last 5-8 years.
My prediction: R19 will be a huge disappointment and people will have to wait yet another year for performance enhancements / UV tool enhancements. I also predict that Studio owners paying the annual US $650 for the MSA might be asking why they are paying so much more when the Prime version includes 90% of the new R19 stuff but at only US $250 for the MSA.
Look out for MAXON making a big deal of the inclusion of the AMD Prorenderer in R19 that's already been announced. This will be the major enhancement for R19. Then of course there will be the usual swag of minor enhancements.
BTW I haven't upgraded to Modo 11.0. Nothing much in that for me. I'll wait until 11.1 and see what that brings.