Does someone want to explain to me how OP_RETURNs increase the blockchain growth, hardware requirements, or otherwise increase the cost of running a node? I keep seeing that concern, and that seems completely counter my expectations. I'd love to see a detailed explanation of how that follows.

If you want to get more background on what the mempool is all about, Gloria and I wrote a ten week series in Optech two years ago:

Bitcoin Optech

Waiting for confirmation: a series about mempool and relay policy

Copies of all published parts of our weekly series on transaction relay, mempool inclusion, and mining transaction selection—including why Bitcoi...

We also discussed each part of the series in the Optech Recap, if you prefer listening (look for the headphone symbol).