While I agree that any “adult” Bionicle sets need to be a step above the GWP, I don’t think that necessarily has to mean Technic. Just look at all the “adult” sets that Lego is making now: they all focus on detail, and I think that’s what an “adult” Bionicle theme needs, more than any specific building system.
I think it really depends what you want to get out of a UCS-style Bionicle set. If you want detailed inner workings/biomechanical layering, then System is the way to go; if you just want a large, visually impressive design (hopefully still with good shaping, of course), then Technic would be better.
In all honesty, the best results would probably come from a fusion. For example, my Technic Bomonga (and, heck, a lot of my other regular-sized builds, too) still uses a fair amount of System to fill gaps and create details, and making a similar model with Lego’s current design philosophy would require replacing all the G1 Bionicle bits with (hopefully) further additional System, as Technic can’t really replicate all those little greebles.