Ad revenue very often dwarfs the income from subscribing users by a huge margin. Sure, a single user subscribing pays for themselves plus a little extra, but your free users make up 70-80% of your revenue
The amount of people that subscribe would likely not pay for the ongoing maintenance costs unless he's willing to work for shit pay. Every hour he puts into maintenance is an hour he didn't put into maintenance the other version that actually pays well enough
Sync for Reddit wasn’t just created yesterday. There comes a point where almost all software is stable and will run be fine without updates indefinitely. Established Reddit clients were at that point years ago.