Anything *aaS is a perfect candidate for subscription. Most other things aren't. Why should you pay recurring fee for what can easily run on your local machine? On the other hand, the $1 price on AppStore for most apps is ridiculously low.
The line is somewhat arbitrary between SaaS and self-hosted. How many software products must truly be SaaS from an infrastructure point of view? Few I'd say. But there are other advantages of SaaS, for both customer and developer: No installation required, no updates to install, easier to maintain from a developer perspective.
