Hmm, yes. At the time Eris was named, there was the possibility that Eris would be classed as the 10th planet, so for continuity with the other planets, Greco-Roman was good. But now all these trans-Neptunian things are dwarf planets anyway, Eris/Makemake/Haumea - so even if one turns up that is bigger than Eris, I think you're right: the 'is this a planet' issue won't crop up in any serious way and pressure the choice of naming mythology to be Greco-Roman, so we'll keep the mythological diversity.

*is happy*