All you're saying is that you haven't used any 3rd party glass in the last several years. There is a ton of stunning 3rd party glass around. You can get lenses for the same or less than Canon's budget glass that performs 90% as well as Canon's very best L glass. In some cases, it matches L glass.
If you have infinite money then sure, buy all Canon/Sony/Nikon glass. Or buy all Leica glass. Whatever floats your boat. But the reality is that when viewing a processed & printed photograph from a normal viewing distance, 99.999% of the population (if not 100%) won't be able to tell if you spent $2.5k on Canon glass or $250 on Viltrox glass. If you pixel peep, you can tell. So what's more important, the print or the pixel peeping?
Hmm 90%....exactly my point. When friends and family look over my website, I never hear those words "gee I could take that with my mobile". I also never hear...I bet that was taken with a tamron or a sigma lens. I would suggest to you that my older EF glass is superior to most current 3rd party lenses in terms of sharpness, colour balance, weather sealing, robustness / build quality, AF accuracy and speed. Sure a 3rd party lens might match a few of these high bar quality specifications but not all. With Canon you get a "full" package and with it comes reliabilty and re-sale value. Some of my lenses I've owned and used professionally for well over 15 years.
Here's a list of my current lenses and you decide:
EF 8-15mm fisheye, EF 11-24L, EF 24-70 f2.8 II L, EF 70-200 f2.8 LIS II, EF 100-400mm f5.6 LIS II. That's the zooms.
EF 35mm f1.4 II L, EF 85mm f1.2 II L, EF 100mm f2.8 Macro LIS, EF 135mm f2.0 L, EF 400mm f2.8 LIS II. These are my primes and there's also the two mk III teleconverters.
With this portfolio, when mated to my R5 and R6ii, I literally want for nothing. I have bought into the Canon eco-system over a long time and I have slowly investing in my glass. However, if I wanted to upgade a specific lens (the RF10-20 is very tempting) all my upgrade or side grade options are found in Canon, not 3rd party. In fact with my current lens choice, they even give Canon's RF offering a hard time. Yes, some of these lenses are quite old and that shows the reliability and engineering of Canon.