I would agree...specifically from the idea that the 1.4's replacement is a 1.8...If you wanted thin DOF and good IQ, the 1.4 was a great mid point - take that away and that will force many to bite the bullet and snag the 1.2 (I know in my case, if the only option was the 1.2 or a 1.8 with IS for $600...I'd just find myself a used 1.2)
I don't think so because the current f1.4 is pretty lousy between 1.4 & 1.8 - so much so that it looses the benefit of the ultra thin DoF. Also the mechanics of the focus is pretty inaccurate for that DoF.
If a 1.8 was really good at f1.8 with accurate focus mechanics then I think it would bs justified as a replacement for the current and ancient 50mm f1.4.
Actually when you look at lens design it wouldn't surprise me ( or disappoint me ) if the new 50mm IS is actually f2 in order for Canon to make it stellar wide open.