I think you're missing the factor that expanding the API surface area, however attractive/productive/cool each individual expansion/feature might be, in aggrega

I think you're missing the factor that expanding the API surface area, however attractive/productive/cool each individual expansion/feature might be, in aggregate becomes a moat maintaining the incumbent's dominant position. So as far as Google is concerned, the implied manpower requirements are a feature, not a bug. Chrome's surface area is now huge (and still expanding at a fair clip), but matching Chrome's featureset has become table-stakes for being considered a viable desktop browser, and independently implementing (and maintaining) that entire set of features requires a correspondingly large developer team, even if you were somehow more labor-efficient.