Makes sense, but I'd stick with the 'elerium is gone' notion. Still, however, you have things like Barracuda - no Elerium but obviously an insanely powerful tech, likely based on refinement of Alien Alloys and other non-elerium techs. So X-Com should be definitely able to field at least lasers, if not some toned-down Plasma weapons (although Apocalypse suggests that humans weren't really interested in advancing non-Elerium plasma tech). Hell, Gauss is said to be such a elerium-less plasma-based technology (and it sadly sucks).
Combining elerium and zrbite makes little sense IMO, both are basically power sources so it makes little difference which one you're using in the end, all is in how the energy is being transported from one object (source) to another (projectile). Zrbite seems to be superior, though, as it can be manufactured (by the aliens at least) while Elerium quite definitely cannot.
