You're actually making the opposite point that you intended: the existing AI-browsers made by The Browser Company/Perplexity had their first mover advantage, but they have little inherent lock-in and are now less likely to retain users since a more notable competitor now exists.
They could make chromium closed source starting from the next patch version while the source of old versions are still available. Currently there is only one company that is capable of maintaining chromium's entire codebase, and that company is called Google.
"But where's the moat, but where's the moat", cries the armchair engineer with a PhD in React.
Meanwhile OpenAI goes brr ...