Interesting myth without much basis in fact. In many ways, Sony and Pentax have far better old lens availability and compatibility than Canon and Nikon.
How many 25-year-old lens models can you buy for under $100 for Canon and Nikon that are image stabilized?
I'll give you that one when using a cheap old lens on a newer in-body stabilized camera. As well some of the older lenses for these makes can be used on newer electronic bodies (but usually without AF which makes the lens impractical for moving subjects). However, newer lenses generally perform better optically in having aspherical elements, have faster af, better resolution more suited to higher MP dslrs than film slrs, better lens coatings like fluorite to reduce glare, less field curvature, etc. The only thing I prefer on some older lenses that I have used is a bit more solid feel in the build (more metal and less plastic but more weight like comparing original metal mount Canon EF 50 f/1.8 vs current plastic MK II version).
However, in my many years associated with photography I feel that I made the right choice in choosing Canon as my photographic system (and I feel the same thing about Nikon) for several reasons which are all based in fact.
1. Any third party vendor that builds adapters, flashes, triggers, cables, etc. will generally make the Canon version first, then Nikon with others a maybe if at all. With Canon you generally get new third party lenses first as well as those companies tend to target the largest possible customer base first.
2. It's easier to find used gear for Canon or Nikon at resale websites such as Craigslist, e-bay, Kijiji, etc.
3. It's easier to rent Canon or Nikon gear from places like LensRentals, local camera stores, etc.
4. If you lose, break or forget to pack something on vacation or while travelling then it's a h*ll of a lot easier to get Canon or Nikon gear somewhere close by (maybe even at a big box retailer like Best Buy).
5. It's easier to find friends or other photogs nearby with similar gear that allows you to swap lens while on a photowalk, etc. to try something out.
6. Almost every photography course I have taken has gone into how to setup Canon and/or Nikon gear through specific menus but the instructor was never certain how to deal with the other brands.
7. These major brands have a complete modern camera ecosystem available now. If I buy a Sony or Panasonic then I'm hoping those companies follow their predicted lens roadmap over the next few years to give me all the lens I need to compete with what I currently have with Canon.
In the end there are people who while using a pinhole shoebox camera take a better picture than me with a 5D3 setup so if you're good then it doesn't matter what system you choose. However, that doesn't stop me from recommending a major brand to someone who isn't hardcore as it's just easier to get them started with components that are sold together and work with no problems or extra effort on their part.