> There are tons of unicellular eukaryotes

But no multicellular prokaryotes. You're right that eukaryotes can be single celled. But my hypothesis is that multicellular life is rare because eukaryotes are rare.

The second half of my comment still applies then. I think Earth-style eukaryogenesis is probably just one way to achieve the energy budget and complexity needed for complex multicellularity... bearing in mind, of course, that colonial bacteria that collaborate for survival definitely exist, spirulina for instance, stromatolites, and all the gross things that form biofilms.