Raw bytes of data (probably incl CSAM) are already on your hard drive, because people have been putting these into witness for years as inscriptions, since these bytes are actually discounted in terms of fees. 
E.g. Transaction: 521f8eccffa4c41a3a7728dd012ea5a4a02feed81f41159231251ecf1e5c79da - mempool - Bitcoin Explorer https://share.google/yr7OQxAgC6XNThp7w
You're missing the technical difference in visibility.
Now: Arbitrary data is buried in witness fields, requiring special software to decode and view.
With 4MB OP_RETURN: The data sits plainly in a standard output. Any node parsing the chain can see it without special explorers.
It’s the difference between data being technically present versus being unavoidably visible.
And that's a huge difference. Don't sugarcoat it.
Is core going to parse this automatically? Is there like an image preview in core GUI or something
No, Bitcoin Core won’t parse or preview images, even with 4MB OP_RETURN. But with the larger limit, you could embed big text art or ASCII images right in a transaction, making complex visuals from letters and symbols possible on-chain, way beyond what was doable before. Only actual images/binary would still require extraction to view as pictures; however, it's only logical to expect popular block explorers to quickly add binary extraction features for user convenience.
Okay so you still need special software. It’s all 💩 but as soon as I don’t have to see it and it does not stop me from getting my usual txs mined I don’t care.
As a side note I decided to give knots a try for a few days we’ll see how it goes but so far all good.
Since it’s all 💩 and most of it might not even be mined ever I might as well save myself some bandwidth 😄.
