Earlier quoted context omitted.
But independent browsers need this metric. Good luck convincing website owners to test against your non-chromium browser with 0.3% market share.
I'm not tracking what's currently going on with the underwater portion of frontend iceberg. Are things still like it was in late '00s and early '10s, where browsers still had plenty of their unique implementation quirks and non-standard features, and plenty of sites were relying on those? Back in the day, it was not entirely unheard of having two significantly different frontend implementations - one for IE, another…
You should ask that how many of the standard features are brought by someone else without huge influence of Google. If you cannot get anything new, when it is enough that Google says "no", then in reality there is only one browser.