The best case for FF MILC would be if FF DSLR sales diminished somewhat while MILC sales rocketed. I think the EOS R is a pretty compelling camera and at its price point, has the best chance to make this happen of the three lower-end FF MILCs.
i don't see what Canon EOS R has over Nikon Z6 (overall at least as capable, less expensive) or Z7 (more capable, more expensive) or Sony A7 III (more capable, less expensive) ? Except for existing Canon users with glass.
Adoption of mirrorfree FF is solely a function of supply. While supply has improved dramatically now - as finally ! there are mirrorfree FF products from 5 brands - there are many caveats, especially when it comes to price/value, that have negative impacts on transition speed:
* sony a7 iii - good price value for body, not for lens lineup (FE glass way more expensive than Canon EF and Nikon F lenses) plus still some ergonomics/UI issues
* Canon EOS R - not a mirrorfree 5D V or at least 5D4 equivalent, but only a mirrorfree 6D III at a 40% higher price than 6D-class mirrorslappers and 25% higher than competitors (Z6, A73)
* Nikon Z6 - about equivalent to D750, but 40% more expensive / Z7 - less capable than a D850, but same high price
IF camera makers would pass on at least some of the cost savings they make with highly automated mirrorfree camera production compared to traditional, labour-intensive mirrorslappers .. and had priced A73, EOS R, Z6 commensurate with what they really are: "entry level mirrorfree FF cameras" ... around canon 6D / Nikon D610 class ... around USD/€ 1500 mark ... transition would happen very, very rapidly.
but even with the makers trying to use mirrorfree FF as another opportunity for their typical, excessive (oligopolist) profit making, mirrorslapping will come to its end soon (enough). for mainstream FF products within 2 years. even when a 1D/D5 class slapper may be for sale another few years.
everything else with mirror a-slapping may see one last, final iteration and that was it.
in Canon terms:
5D5 - probably, 5D6 - no way.
6D3 - maybe, 6D4 - no way.
90D - probably, 91D - no way.
7D3 - surely, 7D4 - dont think so.
