I don't get your point. There will obviously be cased where API's are not needed or too complicated to maintain, which was presumably the case with Firefox, which BTW is a locally installed platform. I don't see how this affect the general discussion regarding (mostly) web platforms - which usually run on a client-server model anyway - maximizing their API footprint to expose the most functionality possible via their API.
Just because you or anyone working there doesn't see a useful use case for using that functionality over the API doesn't mean there won't be someone who will come up with something useful based on that in the future.
Just because you or anyone working there doesn't see a useful use case for using that functionality over the API doesn't mean there won't be someone who will come up with something useful based on that in the future.