We had a hint of that when the RF 70-200 came out and a bigger hint when the prototype RF TCs pictures came out months ago. The RF 70-200 is nearly the length of the RF 15-35 or 24-70, and it can fit in the camera bag on its end like the other RF 2.8 zooms. That's not a small thing. The RF system is lighter than the EF because the 70-200 and 100-500 are smaller/lighter than their EF 70-200 and 100-400 counterparts. The disadvantage is cost. For people that have both the 70-200 and 100-500, the fact that the 70-200 isn't compatible with TCs won't matter much. For those that can only afford one or want one telephoto zoom, then they might have to see what 3rd party manufacturers can do.
There are some disadvantages beyond simple cost. For instance, I personally take photos of wildlife and, to a lesser extent, landscapes. I often go on long hikes with lots of climbing, which sometimes means making a tough choice regarding which of my lenses to bring with me for the day if I can't bring them all. Back in the olden days (pre-COVID), when travel was allowed, I also brought my equipment on small planes with (ridiculously) small size and weight luggage allowances (thank goodness for travel photo vests with large pockets
).
When having to make choices regarding which equipment to bring, a 70-200 with TCs is a very strong contender against a 100-400 or 100-500, as the ability to not use the TCs opens up the benefits for low light and shallow DOF, and the ability to use them helps with reach when needed (which is often). Not having that TC option makes the applications of the two lenses completely different, meaning you are more likely to "need" both. Totally fine when size and weight (and money) are not an issue, but arguably a major drawback for those of us who are carrying gear around for hours on end and who are sick of arguing with airport check in staff about why we refuse to put items made of glass in our checked luggage.