It definitely appears that the EOS R is capable of breathing new life into old and not so old glass, like the TCs.
Through the viewfinder, which is the most important plus, IMO. I can AF at f/16 on my 7D2, but only in god-awful Live View, which is not very friendly to handholding, or even on a tripod with active subject matter, with very narrow angles of view. I'm looking forward to where things like the R are going, but I'm not about to accept its mediocre-for-2018 high ISO noise (slightly more noise than my 6D, and more than my 7D2, when the R is cropped), or give up my realtime OVF for shooting active birds all that quickly. Mirrorless will most likely enter into my picture for the things I use my 6D for now; wide angles and/or shallow DOF.
Canon needs to start considering noise quantity and character again. They have let the 6D2 and R slip in this department (and I fear that Canon won't think twice about releasing a 7D2 successor with more visible RAW noise than the 7D2 at high ISOs). The R apparently uses the same sensor as the 5D4 (maybe the color filters are a little different), but there is more spatially correlated noise in the R than the 5D4 both at high ISOs and in base-ISO shadows.
It is painful to think about how expensive some of my optically poor wide, fast EF lenses were (24/1.4L, for example). US $800 for lenses that are as soft at f/1.4 as they are at the highly-diffracted minimum aperture. The 24/1.4L is a good solution, though, to a camera with highly aliased video, as the lens has anti-aliasing built in at f/1.4.