In terms of third party lenses, I found over the years the following:
- Optical quality tended to be equivalent to the better non-L Canon lenses. For example, the gold ring USM lenses. Eventually Sigma really stepped up its game and produced lenses that specifically solved Canon flaws (UWA coma, for example) and/or generally competed with mid-tier L quality (24-105L, for example). I have over time used (and sometimes owned) lenses from Sigma, Tamron, Tokina, and IRIX (non-cinema).
- Third party compatibility has greatly varied over time, an issue which continues through today. The USB docks helped, but never assured a guarantee. It really made the investment of cash into more expensive third party lenses ($1k+, let alone $6k+) a questionable investment over a decade. In the latest example, something in cameras after the R6 / R5 broke IRIX compatibility — and all that lens series does is confirm focus plus transmit metadata (e.g., lens type). USB firmware docks on the Canon side are largely discontinued, and so with that an inevitability of camera compatibility breakage.
- In the modern context, quality third party lenses tend to be priced similarly EF lenses anyhow. A solid Sigma costs the same as a solid EF comparable, and either one will have tradeoffs in optical quality, DLO integration, or lens barrel robustness.
- Very few compete optically or in lens body construction with modern L RF designs.
I've yet to encounter any third part lens that competes with premium contemporary Canon lenses, and I've yet to have a friend say they have either. If someone is willing to spend $10k in any currency on a single lens I sincerely doubt the lack of third party lenses are going to cause said person to jump ship because of a missing third party line — or it's a rare, very niche situation. Canon makes a premium product that is as expensive as "L" because it caters first to professionals.
In an older IT company I worked at when getting going in the industry the president used to tell us that we had one flagship product for a market and all other products just needed to be technically eligible — that is, tick enough boxes on paper to be allowed to compete for client funds but by no means had to be better than anyone else's (or even half as good). Canon's non-premium tier probably meets the "technically eligible" definition in a modern context of other manufactures. For example, the STM non-L lenses get the job of focusing photons onto a sensor done and they're fine for what they are but they're not the reason for someone to start with Canon cameras as opposed to Sony or Nikon; rather, they make a Canon camera body equally eligible for a newcomer's cash.
I think from Canon's perspective the EF catalog, which is still present for common use cases, is a de-facto third party lens option for the RF mount. Want to save money? Sigma has a 12-24 f/4 but Canon EF has an 11-24 — align sales and they're the same effective price in Canada. Sigma has a 24-105 f/4 but Canon has an EF 24-105L II IS USM — align sales and they're the same effective price in Canada. Ditto for Tamron, and ditto for IRIX (but in IRIX's case Canon also has AF). Might some of these be optically better than EF? Sure, but there's also no assurance they'll work on the R1 II, or the R5 III — but the EF lenses
will.
All of my third party lenses have been sold off, and they were all sold off for compatibility reasons. I won't buy more until they are officially supported by Canon. Not while EF exists new or like-new with warranty, at any rate.
That stated, I think the people in this forum buying $10k+ lenses need to keep in mind that many people want the experience of exotic lengths but cannot justify the lofty price of new editions. And for mediocre non-L products (not bad, just average for the industry all factors averaged out) then why not cheaper third party options? Not everything is brute force making money or having the most Instagram followers— it might be nostolgia for 1990s photography (enter Sigma's more affordable Great White or prior Great Blacks). B+ and A- for 2/3rds the cost for most people will be perfectly fine — they probably aren't editing anything so refined that A+ would make the difference anyhow.
Myself, I probably represent an aspect of this category. I am well heeled thanks decades of hard work and good fortune, but I don't buy the $10k lenses. Why? I have a farm and animals are expensive; I am an enthusiastic power lifter who spent loads of money building a sweet home gym; I like to travel; etc. Eventually I want to retire. My wife has her priorities. A- for me is good enough as a (generally) non-commercial photographer, and if Sigma offered select lenses under Canon blessing then I'd think about them in a serious way. Jump ship without? Obviously not. Consider them as alternatives to B through A- RF lenses? Sure thing.
For people much less fortunate (industry specifics, now retired, many mouths to feed, injury with medical needs in an expensive health system, etc.) these options actually might make the selection of a platform a real material matter. This being a Canon forum, I don't think it's good enough to say just go buy Sony or Nikon — obviously that's an option — but the nature of this forum is hope for the future for the Canon community. I think that just like in the EF era third party options form entry points, retirement enjoyment, etc. I don't think people should sneer as hard as they do at the possibility. Just because many of us are fortunate enough to have all Canon gear doesn't mean that's the only valid path forward. And even those of us with non-Canon gear (some of my gear is solid but not Canon) we'd like to replace them with non-$10k but better than meh options that are Canon-compliant, and if a third party makes that a better likelihood then hurray. I mean, $4.1k for a 500mm f/5.6 with weather sealing from Sigma (or their $7.7k f/4 EF mount edition still in stock) vs $11.5k for a Canon EF 500 f/4 for your average non-commercial photographer? Only an ignorant person would tell the candidate to avoid the Sigma and cough up an extra $7.4k for... what? I can't think of the reason.