1. Increasing the size of OP_RETURN allows CSAM to be stored in a non obfuscated way
It's already possible to search and view arbitrary data with website that show OP_RETURN and Ordinal data. But running full node software alone wouldn't let you search or view any arbitrary data.
2. Arbitrary data presents "unknown unknown" attack vectors, some are going to be immediately obvious, like malware, but we should expect bad actors to be more clever than even us.
FWIW, it's possible to add arbitrary data with size bigger than 80 bytes using OP_RETURN if you can get miner to submit it to their block directly.
3. If it causes the average block size to increase, it could strain nodes pointlessly.
I get the concern. Although looking at block weight size in past year[1], block usually reach it's maximum allowed weight size.
[1]
https://statoshi.info/d/000000002/blocks?orgId=1&from=now-1y&to=now&timezone=browser