It's interesting if you look at releases over the last five years to see what Canon's priority on lens mounts are:
EF Premium (L series, TS and DO lenses): 12 lenses released
EF-S: 7 lenses released (but mostly minor revisions)
EF-M: 5 lenses released (all new designs)
EF full-frame non L: 3 lenses, of which only one (the 24-105 f/3.5-5.6 IS STM) is a 100% new design, the 50mm f/1.8 STM is the old 1.8 with a new STM motor system, and the 70-300 f/4-5.6 IS II USM is essentially an upgrade of the previous 70-300 lens.
So, in the last five years, non-L EF full frame got one new design (for a kit lens) and two very good but hardly significant updates to two of their most popular lenses.
No doubt there's a market for an improved 50mm f/1.4 non-L but I'm pretty much certain if they do a 50mm f/1.4 IS it will be a premium L lens, not a cheaper model.