If Comcast's customers (big corporate ones, they don't care about regular people) complain, then Comcast would probably override the RPKI validation too. Well, unless they're somehow legally precluded from doing so, but in that case RPKI hardly matters since they couldn't peer with them even if their routes were valid.

They'd be connected to a non-sanctioned network that was connected directly or indirectly to rostelekom. For instance maybe Arelion, which is Swedish. The internet isn't yet in the situation of the financial system where the whole world obeys US rules out of fear.