But their research costs are extremely high, and without a network effect that revenue is only safe until a better competitor emerges.

You're moving the goalposts, given the original complaint was not about research costs but about the marginal cost of serving additional users...

I guess you'd be surprised to find out that Meta's R&D costs are an order of magnitude higher than OpenAI's training + research costs? ($45B in 2024, vs. about $5B for OpenAI according to the leaked financials.)