Cheaper IS implementations can be very cheap - just look at the £85 18-55 IS lens. I see no reason why another all-plastic replacement for the 50/1.8 line couldn't be released with IS for a similar introduction price - whether its f1.8 or f2.
I have a feeling the higher end f1.4 and f1.2 lens replacements are less likely to get IS, but it'd be nice to be proved wrong - those that don't want the feature can always leave it turned off, as long as they afford it.