Yeah for historical links it makes sense to fall back on IAs existing archives, but going forward Wikipedia could take their own snapshots of cited pages and substitute them in if/when the original rots. It would be more reliable than hoping IA grabbed it.
Shortcut is to consume the Wikimedia changelog firehose and make these http requests yourself, performing a CDX lookup request to see if a recent snapshot was already taken before issuing a capture request (to be polite to the capture worker queue).
Ironic, I know. I couldn't find where I originally heard this years ago, but the InternetArchiveBot page linked above says "InternetArchiveBot monitors every Wikimedia wiki for new outgoing links" which is probably referring to what I said.
I didn't know you can just ask IA to grab a page before their crawler gets to it. In that case yeah it would make sense for Wikipedia to ping them automatically.
Why would they need to own the archive at all? The archive.org infrastructure is built to do this work already. It's outside of WMF's remit to internally archive all of the data it has links to.
Anyone can request anything be removed and they may honor the request: https://help.archive.org/help/how-do-i-request-to-remove-som... they say nothing about only removing things illegal in the US or anything like that, meaning they can and will remove things based on personal judgements about whether it should be archived.
https://meta.wikimedia.org/wiki/InternetArchiveBot
https://github.com/internetarchive/internetarchivebot