PoW coins have a multi-year headstart, and Bitcoin didn't have any truly interesting decentralized competition until Ethereum. Ethereum is moving to PoS, I presume because being able to more cheaply and quickly execute smart contracts is deemed more valuable than artificial scarcity. A sentiment with which I could not agree more.

The problem then becomes how do you determine the 'real' Ethereum. For Bitcoin, it is generally understood as the blockchain with the most proof of work.

With proof of stake, what's the disincentive to participate in all forks, or even a large number of alternate histories. If there are multiple Ethereum blockchains using PoS, how can I as a new user determine which one is 'the' Ethereum?

The one with the most utility (connected services, ability to spend, and other infrastructure).

Ethereum is going to PoS as a mechanism to reduce liquidity and increase the coins price. Nothing more. The entire history of ethereum is full of a lot of... less than ideal decisions like this.

Whether or not that's true, I literally have code sitting on the shelf waiting for it be cheap enough to run on ETH, and I think PoS will make it cheap enough.

Ethereum is moving to PoS because it would greatly help with scaling. Once Ethereum has Casper, they can start thinking about implementing sharding allowing on-chain scaling.