I think you overestimate the percentage of Chrome users who use Chrome for the extensions rather than other reasons ("it's fast", "it's what I'm used to", "I like Google", "Firefox is weird")

There's no way that any of this is going to meaningfully affect Chrome's market share.

maybe what we're really seeing is an alternative - users don't care enough about the quality of the extensions ecosystem. Otherwise browsers should be racing to have the best extension ecosystem.