Satoshi said it himself in the Bitdns debate (which tried to spam the chain with domain names in 2010):
"Piling every proof-of-work quorum system in the world into one dataset doesn't scale. Bitcoin and BitDNS can be used separately. Users shouldn't have to download all of both to use one or the other. BitDNS users may not want to download everything the next several unrelated networks decide to pile in either."
Core has done this very thing to a more extreme extent.
Arbitrary data is anything non financial. It’s not a mystery, and allowing for any other use case on BTC weakens security by forcing nodes to foot the bill and store the data (SSD is at historically high prices rn). In other words : SPAM DOESN’T SCALE
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https://thebitcoinportal.com/live/spam/overview