We're still stuck with the model sets created back in the '90s. The chances of GW ever releasing a multipart Guardsman set...?
I'd be interested to know how popular these armies are and where they rank on the 'most bought' list.

I assume that the chances of seeing new Guardmen is directly linked to how much money GW can milk from it. Which is why we'll never see new Sisters of Battle units.

Well if you look at most armies basic troops, the models themselves are practically the same, they have just been updated to include extra parts, for example CSM have pretty much the same models, just a couple extra spikes added to the special weapons and teh addition of a standard bearer.

The Imperial guard I think have aged fairly well (or at least the plastic ones have) if there are going to be new ones I would expect plastic variants of vostroyans and steel legions etc. (the versions still in metal).

If its plastic already its unlikely to be updated. If its metal then they will almost definitely release a plastic version now if there isn't already a plastic alternative to fill the gap. SoB will get a plastic range if they ever release a full codex, its more whether there will ever be such an update.

GW seems quite content to empty their storehouses via the latest and bestest Formations anyway. They could resculpt stuff which takes time and money, or they could blast out some rules that may or may not contradict the BRB and may or may not be the best thing ever on the tabletop.