Well in this scenario, miners have decided to steal from the 'shitcoin' sidechain to make room for a 'good' sidechain. The 'shitcoin' ledger would become valueless, and shitcoin holders would be out of pocket.

That is the point though. If a vanity/shitcoin takes one of the drivechain slots, it risks getting evicted and liquidated by Bitcoin, if miners (and indirectly the users) have decided the shitcoin didn't deserve that space. So, sidechains must necessarily be 'good' for Bitcoin, or the sidechain will be sent packing.

Want to start a side chain? Think long and hard about how it will synergize with Bitcoin. Put your idea into public and make sure it gets broad community support (e.g. BIP) before trying to deploy it. If you do those things, you'll have a good idea if Bitcoin will evict+liquidate your sidechain or not.