I think you can still do a one-time purchase app with support for the next X versions and still get the revenue model of a SaaS. Stop thinking about the app as the final product and start thinking about it as a platform.
Sell email client for X, then build additional plugins for client that solve a problem for a subset of users and offer that as an upsell. You avoid bloating the original app for those that want a sleek solution but offer enough plugin functionality to let users tailor to their purposes.
And each of those plugins can have a paid upgrade cycle as well, a long with the ability to bundle plugins with app for "Enterprise" or "Power User" price points.
Sell email client for X, then build additional plugins for client that solve a problem for a subset of users and offer that as an upsell. You avoid bloating the original app for those that want a sleek solution but offer enough plugin functionality to let users tailor to their purposes.
And each of those plugins can have a paid upgrade cycle as well, a long with the ability to bundle plugins with app for "Enterprise" or "Power User" price points.