Wouldn't this be the worst of both worlds for the 3rd party app developers? In the current model, the developer of Apollo could charge $2.99/month and pay themselves $0.47/month/user. If we assume 80k paid users, that's $450k/year - not bad for a passion project!
If Reddit instead took it out of developers hands and made users of Apollo pay Reddit for access, that would seem mighty unfair to the developer of Apollo, wouldn't it?