There really isn't a better way to run a business (even if it's just a single app) than to get someone to pay you an amount on a fixed interval. It's predictable and generally weathers shocks pretty evenly. This is also why for most investors dividends are a big selling point - consistency.

Even Apple somewhat relies on this, except it's been in 2 year intervals. It's the point of so called "planned obsolescence."

So if you're an indie developer, you only have a few options.

1.Go through the rat trap of doing ads and trying to grow a huge userbase with marketing etc..., which everyone hates but most tolerate because it's free.

2. Charge a 1 time fee and hope that you price it right to support long term development and sustainment, knowing that you'll probably limit your market significantly.

3. Charge a recurring low(er) fee and draw a balance between losing a lot of very price sensitive people and having to run a marketing campaign and ad system.

No good option there but subscription seems to have the most benefits. By the way, everything needs upkeep.