I'm curious – what great 3rd party lenses are there for which there are no Canon equivalents? The optically stellar manual focus Zeiss lenses come to mind (although there are Canon L-series equivalents, just slightly less optically impressive and much cheaper). There are a handful of 3rd party options for lenses like the TS-E lenses (Schneider) and MP-E 65 (Venus, Mitakon), but again, Canon has versions.
The converse seems more true, for me at least – there's no 3rd party 600/4 for Canon, no 3rd party 11-24 or fisheye zoom, no TS-E Macro lenses.
Hm, okay, maybe not many lenses, but certainly quite some some.
Until recently Canon got no 50mm that was as good as the Sigma 50mm Art. The 1,2L was noteable less sharp and slower.
I think there was no wide angle with IS and f 2,8 like the Tamron 15-30 f2,8 VC which I used a lot for a lot of beautiful, handheld shots in low light.
Laowa got lots of unique lenses. The 24mm macro Probe is certainly VERY unique. Also they got the insane 15mm Macro (unique look) and some nice 2X Macros (Canons Macro was only 1X I think?). The Zero Distortion 12mm also got a lot of good feedback, a light and sharp Super Wide lense.
Tamron got the first 35mm IS lense I think. The Sigma and Tamron 150-600 was also very popular, I am not sure if Canon got a comparable lense?
Cinema lenses is another world, I guess there are plenty lenses to be found. Also vintage lenses which are adaptable are very unique.
These are not all "irreplaceable", there are at least similar options from canon. But its a nice addition. Laowa certainly is unique.
And whats also very important for the market are the affordable lenses. You are correct, Canon offers everything a photographer actualy needs (especialy high end lenses like the 600 f4 you mentioned) - but the competition certainly produced some very good and affordable lenses. The Sigma Art Series for example even beat some high end L lenses in image Quality (50mm, 24-104 at the time, 105mm and 135mm come to mind which are all extremely sharp) AND untercut the price by 50% or more at the same time. Same goes for Tamron G2 Series, Thats a very welcoming set of options.
I think a bigger market and more options/competition is always good for the customer =)