Sigma's 50/1.4 A won't 'force' Canon to do anything.
Correct. I'm pretty sure that lens design and manufacture is something that is planned out many years in advance.
They'll likely come out with a 50/2 IS delivering very good IQ, priced around $800, and that lens will probably outsell the Sigma. The Canon 50/1.4 will likely continue to outsell the Sigma, and the 50/1.8 II will far outsell both. Cheaper sells.
So long as Canon continue to make the cheaper models, yes...
If Canon updates the 50/1.2, which is probably a long way off, it'll remain f/1.2.