It's not just that the DNC can't afford to take out ads on social media, it's that social media can't just give them free ad spots without breaking the law.
So if the DNC can't afford to buy campaign ad spots on social media and social media giants can't give them away to the DNC without being heavily fined and having C-levels tossed in prison then they'll just ban campaign ads so the RNC can't buy them either.