After coming to Canon from Nikon after years away from the hobby, I got fed up with the variable QC of Canon glass - despite their supposed 'premium' stance and cost (within the DSLR 'box').
This 'buy & try' method of fnding good copies of supposedly pro-level glass combined with the ski-slope depreciation curve of DSLR values post-purchase (not a Canon-specific issue), led to a radical change in my gear cabinet.
I sold my 5D and all my Canon glass save a nifty-fifty. Replacing it was smaller selection of top used Leica R glass (no duff copies yet seen and rarely, rarely heard of), an R8 (already fully depreciated), a used Nikon 5000 ED scanner, some EOS-R adapters and the cheapest DSLR body I could find that would do the job, spot meter and take a Leica adapter - a 30D.
One can only hope that Nikon's suprising come-back with the D3 will give Canon the kick in the ass needed to overcome it's (natural) 'I'm King-of-the Hill, take it or leave' complacency (sic).
That beig said, there is only so far you can go with factory mass-produced optics designed for a high-iso, AF, USM-happy and in many cases, JPEG-only market.
Canon (or Nikon) will only aim as high as the average customer's expectations.