1) many pro photogs never use the telephotos. Simple reportage, travel, wedding, stock, etc. are probably fine with trinity zooms. The 2.8 trinity is already announced, with IS all the way. Many working pros don't need anything more. I grant there are other pros that almost only use the telephotos (sports, wildlife), but it's not like Canon can't get any sales until they have literally everyone's bases covered.
2) of course all EF glass works via adapter. I think pros are probably OK with this as long as it doesn't fail (break, weather, etc.) I mean pros think nothing of a macro tube when needed, and no-one using the white telephotos would refuse a teleconverter. So why refuse the RF-EF adapter?
I actually feel point 1 strongly enough that I wonder if they made a mistake not releasing the R with trinity 2.8's. But they're pros at marketing as well as optic design, so I imagined they have this all timed out. The current R's, I bet, aren't even being made to sell, but rather to get people talking about the system. No other motivation for the ridiculous 28-70/2.0... So, if this year is just to build buzz, I bet we'll still have trinity plus a pro body all long before the Olympics, so all the reportage going on can be done by people who are totally familiar with their R's, even if the actual event shooters are still using EF glass (and potentially EF cameras).