Even though I'm no longer into birding
,I still need a longer focal for landscapes or the odd bird/animal. No tele prime offers the same versatility as a tele-zoom, in my case the EF 100-400. Especially when hiking in the Alps, I really appreciate not having to change lenses, often not even for a macro. If I didn't have (and love) the EF, I'd sure buy the RF 100-500, an even better lens according to your testing.
I believe Canon are right when replacing or completing their big whites with zoom "equivalents".
PS: The EF 100-400 and the TSE 24mm II were for me the reason to go Canon, and not Nikon...no regrets!