It's still a common lens combo on some of the wild life / irish sea birds workshops I go on. It's more than sharp enough for a R5, it's an amazingly light weight 600/5.6 and it's AF and IS are excellent too. I think it's price on the used market makes it even more appealing. I see similar results with my EF 400mm f2.8 LIS II and a 2x TC. I suspect that when Canon move to 60mp + sensors we might see some optical resolution issues with a number of extended lenses, including some of the current RF lenses. I thnk it's still apopular lens because it's a sweet spot and Canon haven't yet improved upon in that use case scenario. Not everyone wants the RF100-300/2.8 even though it's a remarkable optic. It's nearly as sharp as the mkII prime (which is one of Canon's sharpest), it's only a bit heavier and it negates the need for an additonal 70-200/2.8 and camera body.
The Sony 300mm f2.8 is the current state of the art, where 300mm f2.8's are concearned. It's a size and weight marvel. However, the older EF 400mm f4 LIS DO II is comparible in size but slightly heavier in weight. This comparison shows that Canon is behind in the developement of a number of it's super white tele's. While Canon is developing stunning lenses like the RF100-300/2.8, Sony puts or their 300mm f2.8 offering and reminds us all that they have serious capability. Let's face it, Sony's 1.4kg 300mm f2.8 is in the 70-200/2.8 ball park for size an weight.
While Canon are considering big white zooms to give us a versatile wider reach, Nikon have intergrated their larger big black tele's with integrated TC's, which extends the over all reach at the long end. The Nikkon integrated TC's look on paper shaper than Canon's RF seperated tele converters. No one has yet integrated a 1.4x in to a 300mm f2.8, only the 400/2.8. With a larger and heaver lens, the increase in size and weight isn't too detrimental, but on a super light 300mm f2.8 (such as Sony's) it would start to make big factor in the lens' over all weight.
One wonders on the size and weight impact of this kind of integration. If it can be done and if the whole package from Canon comes in at over 2Kgs on the scaled then it's a missed opportunity.
For me, I've often thought about aquiring a EF 300mm f2.8 LIS II as a smaller "walkabouts" super tele when I don't want to take my bigger 400/2.8 out. However, when i need it, my 400 always delivers and the mass of the lens is worth while. It's then that I realise that I already have all my optimal lenses.