> statistical acumen to think he can make a reliable estimate of a monthly average from some span of time shorter than two months

Perhaps because those numbers are provided on the Playstore dashboard? You should question Google's acumen in providing those statistics to developers?

And people have been estimating ARR through projections for a long time.

I already have services running for a decade+ in a product which I posted here: https://news.ycombinator.com/threads?id=faangguyindia&next=4...

In the end, simplicity wins.