Given that Omicron has higher viral loads, shorter serial interval and shorter incubation period than Delta, it is much more likely that Omicron is more intrinsically virulent as well and would be worse if you dropped it on a totally naive population in the summer of 2019. The current population, however, is longer immunologically naive, and I think that researchers are underestimating the existing seroprevalence.
At the same time if there's still pockets of entirely unexposed/unvaccinated people out there in rural areas or something, Omicron will likely find them since it seems to largely ignore unboosted Nabs to prior infection or vaccination. Kids in colleges will be taking Omicron home for Christmas to rural families right about now and a few weeks from now we should find out how many seronegative people are left.
If it Omicron fizzles though it is going to just be that Delta didn't leave anyone immune naive in its wake.