I'm intrigued by your recommendation of Sony/Panasonic.
There are some pros/cons for each system and some missing niche lenses of course but I struggle to understand where Canon has missing gaps from EF/EF-S from Canon/3rd parties and Canon RF lenses. There are a number of people missing the 200-600mm/5.6-6.3 for instance and some Canon pricing (especially in Europe) seems to be unwarranted but lens range overall isn't that much different?
Canon niche lenses like 8-15mm/4, 11-24/4, TS-E, MP-E, 100mm/2.8 (1.4x), cheap telephoto primes etc don't have equivalents as far as I know. Other lenses can be debated for cost/size/weight/quality and yes, there is no RF50mm/1.4
It wasn't my recommendation you were responding to, but ... Yes, particularly with the help of an RF/EF adapter, you can get pretty much anything you might want from Canon. And yes, the RF/EF adapter gives you access to third party lenses. However, third party manufacturers don't seem to be releasing many (any?) new EF lenses, and in any event using EF lenses plus adapter adds to the size and weight of your setup. Whether that increase in size and weight is significant largely comes down to the user (some care, some don't), but having access to newly designed smaller and lighter lenses which don't require an adapter is valuable to some people.
Looking at Sony, there is obviously a wide range of lenses, both Sony and third party, which do not require an adapter. And plenty of the Sony and third party gear is good gear (Canon makes many high quality products but so do others). Looking at Panasonic, the range isn't as broad as for Sony but the L mount alliance does seem to be bearing fruit. And the prices are often better than Canon offers for comparable products. (I realise what people consider comparable products can vary too, eg I consider a 70-200 (or 70-180) f/2.8 without IS as comparable to a 70-200 f/2.8 lens with IS if it will be used on a body with IBIS. No doubt though, some people will say the lens with IS is not directly comparable to a lens without IS.) For example, you can get a Panasonic S5 II plus Sigma 24-70 f/2.8 DN Art lens for under $A5k, while a Canon R6 II plus Canon RF 24-70 f/2.8L IS would be more like $A7k (based on prices I just found online now - there may be better deals out there somewhere). Obviously the gear is not exactly the same so you need to work out the pros and cons of each for your own photography, but the price difference is a big enough to get my attention. And the price difference may well increase as you add more lenses.
Anyway, I do not mean to suggest Canon doesn't make good products, or that anyone shouldn't buy Canon if the Canon system offers what they want. However, for anyone not already invested in Canon (and in fact even for some people who already have some Canon gear), generally speaking I would recommend they at least look hard at Sony and Panasonic, in particular, because of the value proposition, and go with Canon only if Canon offers something they really want or need which the others don't (eg the 28-70 f/2L, they really really prefer Canon's ergonomics, etc) and they are comfortable with the budget required.
