Not GP, and not saying I agree with them, but it may be worth remembering that Netscape had 90% market share at one point. Active user count may not be the moat you imagine.

Adoption of web browsers was also much lower when Netscape was dominant. 90% marketshare is less meaningful if you're only 1% of the way to the potential market size. Peeling away users who talk to ChatGPT every day is very possible, but harder than getting someone whose never used an LLM before (but does use your OS, browser, phone...) to try yours first.

I think the even better analogy than browsers is search engines. There aren't any network effects or platform lock-in, but there is potential for a data flywheel, building a brand, and just getting users in the habit of using you. The results won't necessarily turn out the same - I think OpenAI's edge on results quality is a lot less than early Google over its competitors - but the shape of the competition is similar.