Alternatively, Reddit could roll this out slowly and give Ethereum 2.0 a chance to land. By 2021/2022 when Ethereum 2.0 Phase 2 lands, scalability of Ethereum should be much greater than it is today and it should be a solved problem.

Also, most of the transactions between Reddit users can be done off chain if needed. I'm not sure how they've designed their architecture, but it does make sense to aggregate multiple transactions into fewer, big transactions instead of small microtransactions for every comment.