Why would Bitmain do this when all signs are pointing to Ethereum moving to Proof-of-Stake? I find it hard to imagine them risking the R&D cost that goes into developing an ASIC like this unless they think they're going to make it back very quickly.

That's assuming it's Ethash-specific though. I guess you could also mine Ethereum Classic with it at least (are they planning to move to Proof-of-Stake? I havne't kept up).