DeepSeek is somewhere between Sonnet and Opus in capability, for much lower price.
Their $0.87 per million output tokens is one of the few API offerings that is probably subsidized (the break-even price is probably around $2 judging by[1])
I think Anthropic and OpenAI's margins will erode some over time. But I think they are very profitable now on the API prices they are charging, and their margins will remain healthy.
Put it like this: AWS is a very expensive way of getting compute, and there are numerous competitors that are much cheaper.And yet AWS is very profitable.
[1] https://openrouter.ai/deepseek/deepseek-v4-pro#providers