I am actually pretty curious why EF users switched, or haven't yet switched,
I wonder how many fully committed to an actual switch, at least among users of this forum. I’m absolute terms, probably a high percentage or even a majority of users did that, because the numbers show that most ILC owners have just an APS-C body and the 1-2 lenses that came in the kit, and thus would have bought an R body and kit lens(es).
Objectively, the R bodies offer several advantages over EOS DSLRs...AF improvements, faster frame rates, silent shooting, more information and true WYSIWYG in the viewfinder, etc. There are also some disadvantages...lack of cross-type AF points, potential for rolling shutter (with all but the R3), for some giving up an OVF, etc. On the lens side, there are also advantages (control ring, lower cost consumer lenses, an f/2 zoom, etc.) and disadvantages (slower apertures made possible by image sensor AF, higher cost L lenses).
I suspect most users significantly invested in EF transitioned gradually and in many cases still haven’t ‘switched’. Canon made this easy, the adapter functions perfectly, and can add capabilities (control ring, filters). With the newer bodies, old flashes work with the new multifunction shoe.
Personally, I bought an EOS R and RF 24-105/4L in 2019 to try out the system, but kept on using my 1D X as my primary camera. Once the R3 came out, I switched to that as my primary camera and swapped out several of my EF lenses for improved (for me) RF versions – EF 24-70/2.8 II → 28-70/2 (faster aperture), EF 70-200/2.8 II → RF 70-200/2.8 (smaller and lighter, better IQ), EF 16-35/4 → EF 14-35/4 (wider at the wide end), EF 70-300L → RF 100-500 (not really a direct swap, but I had the original 100-400L that I swapped years ago for the smaller 70-300L, after getting the 600/4 II as my primary birding lens). I recently added the RF 100-300/2.8 for indoor events, and also R8, RF 24/1.8 and RF 28/2.8 mainly for travel.
But I've also kept and use several EF lenses for which there are no RF versions (TS-E 17 and 24, EF 11-24L, and for two of those the drop-in filter adapter adds a significant capability) or for which the RF versions don't add anything I want/need (600/4 II where the RF version is lighter but not optically better and I have no problem with the weight of my current lens), EF 85/1.4L where I don't need f/1.2, and EF 100/2.8L Macro where the RF version has focus shift in some cases and I don't need the 1.4x because I also have the MP-E 65).
On the flash side, I switched my ST-E3-RT for the ST-E10, bought an EL-5, but I still use my four 600EX-RT strobes off-camera.