The question that is unknown is what percent of reddit's userbase uses exclusively third party clients vs those who use reddit's official client. And then, what portion of the third party userbase will migrate to the official client after those third party clients go away.
I'm sure spez knows at least the first set of numbers, and they find the amount of expected loss to be an acceptable risk.
The various alternative reddits that have popped up in the past prove the tech stack isn't complex. Their lack of user adoption shows that the secret sauce is the userbase, not the tech.