My theory is not just giving people a browser but giving them the best possible experience. Chrome has an enormous market share given every OS and device comes with a free web browser. Further the developer experience using Chrome is also superb!
Usability is a very subjective topic, but for me there are things in firefox or it's addons that I cannot have in chrome.
And in the end I need to trust my browser for daily (non-developer) use. Trusting google is naive. I don't say Mozilla is deserving of unquestioned trust either, but out of the two it is the better choice.
In the end my (very subjuctive) judgement is that the performance difference in daily use doesn't outweigh the benefits of not using the browser of the monopolist that makes their money with data collection.
That doesn't mean I don't use the browser or it's engine in other ways tho.
I also think this recent change sucks so it’s not all roses.