I think the likelihood of seeing a T1000 is on par with seeing a Sabertooth from the X-Men Origins line. Probably even less so. Sideshow's was acceptable at $50 with pop on bullet holes and interchangeable hands. That would translate as an R&D nightmare for a company like HT to pull off with any sense of realism without having several aspects of the piece, like the clothing on the arms, sculpted to achieve that perfect blend from sleeve to blade/hook.
If Sideshow's figure covered more, I might agree, but I have a feeling HT would enjoy the challenge, plus sales could do real well if they tackled his damaged heads, the split one, the eye blown out head. Really, those are the only reasons I care for HT T-1000, as you said, the Sideshow one is actually pretty good, but not including those pretty cool and prominent aspects that set the T-1000 apart from a regular Terminator hurts it a bit and I think leaves a door open for HT to put it out and still sell quite well, that and the new comers to the hobby that never got their hands on the Sideshow, I think they'd be missing a chance to make more money with T-1000. I want Sabertooth, but I can sort of understand why that might have been viewed as a gamble and not worth it, the reception for that film seems to be a lot of mixed feelings.