Why should the apps have to pay? Why can't reddit provide a paid version that allows you to access the APIs, which the external apps can then use? You already have to log in to them anyhow so it's not like you'd need an extra API key or anything. $20/year is probably an order of magnitude more then they make on ads served to me.
The weirdest thing about the ad economy to me isn't that it exists, but as I've banged on before, that the companies will cling to it even when people are begging to pay money to get rid of them. These companies make virtually nothing per year in ads and make up for it in scale.
Latest easy stat I can find [1] is $350 million of ad revenue in 2021. Certainly this is less than $2/year from most active users. I'm willing to outbid them by an order of magnitude easily. If even 1% of the user base follows along that's a lot of money.
I don't understand why these companies are so insistent on bending their entire platforms around ad revenue to the exclusion of all else, when "all else" is so much money for someone already scraping pennies from people. Besides, a lot of ad spend was tied to ZIRP; I would think that while the ad business as a whole is certainly in no danger of disappearing, now is a great time for any ad-focused business to be diversifying because spends are very likely to go down in the medium-term, if not the short term. A lot of economic signals are flashing "recession" and ad spend tends to get hit pretty hard in that context. A solid base of subscribers paying real money for your service is a better foundation to go into that sort of economic environment than being solely dependent on ads. (Obviously, a recession will hit your subscriber base too, but that's stickier than ad spend I expect.)
(And an obligatory observation that they are shoving ads in your face, tracking your every move, crafting something you spend hours a week on to increase your advertiser profile value in your every interaction, deploying every addictive psychological trick they can think of to keep you active, and fundamentally polluting at a deep and profound level the entire social sphere of damned near the entire species... for pennies a month. That's all. Pennies a month. They're not even making the money you think they are. They're doing all this to you and the collective social sphere for a few pennies a month.)
[1]: https://www.businessofapps.com/data/reddit-statistics/