Strictly speaking, this can't be true because I have purchased apps (that have been around for years and are still supported) on the Mac App Store that are not available any other way.

Yeah, I guess what I mean is, as a percentage of revenue, the Mac App Store is a rounding error for most developers. There's a small number of bespoke, $5-$15 apps, possibly distributed only through the Mac App Store, strongly reminiscent of the early days of Mac indie development; but this is fractions of a penny relative to the ~11 figures of revenue the iOS App Store drives every year.