Key word here is 'on average'. Most people are not going to spend $25/year for access to a third party reddit app. That leaves the hardcore users who are going to cost the dev much more than $2.50/mo, and when ads are no longer able to be served and NSFW content is also gone where is the profit then?

Putting aside the bad salesmanship and planning of selling a forever license, it is absolutely way too much money.