So the question is... When will Canon respond to the Sigma 50 Art with an update to the 50L?
Canon's lenses have historically had a life of somewhere between 15 and 20 years as a current product.
Expect the 50/1.4 and 50/1.8 to be updated well before the 50/1.2L.
Maybe, but probably not, unless they decide to add IS to one of them as part of an "IS everywhere" campaign. Both are over two decades old, which tells me Canon probably doesn't care much about updating them. They're more than good enough for low-end lenses.
Look at the lenses that Canon has updated in the past ten years and the age of the predecessor:
24–70 L II: 10 years
16–35 L II: 6 years
70–200 L II: 9 years
70–300 (L as an alternative to non-L): 5 years
300mm IS II: 11 years
400mm IS II: 12 years
500mm IS II: 12 years
600mm IS I: 12 years
24mm IS: 24 years
35mm IS: 22 years
Only those last two replaced lenses that were more than a decade old, and both of those were IS upgrades to previously non-IS lenses.