The cost of the third party apps themselves was trivial, and if they just wanted to recoup those costs they could have proposed a much more reasonable cost per user for third party apps.
It’s about control, not about profit.
$10m isn't trivial unless your operating costs are in the billions.
Moreover, at that cost, less than a dollar per user per year, Apollo could have instituted a $1/year subscription. But instead Reddit wanted over $100 per user per year, two orders of magnitude more than the cost.
It's one of those typically short sighted "oh lets remove this thing costing us" without understanding the long term impact to value.
You don't over burden one single entity with large recurring payments (the app developers themselves), your power users provide revenue, and you can slowly work on your value proposition of "hey we have updated our app to not be as crappy, you can browse reddit for free if you switch back".