That's not quite right. The JS source code is committed on chain via a contract interaction and every minted token gets a token hash which, when run through the JS source, can recreate the art.

But...the execution environment is still your web browser and non-animated Art Blocks NFTs still have a "preview" stored off-chain like most other. Neither running the code in your browser or retrieving the preview is an on-chain operation.

it is pretty trivial to retrieve JS code and the hash from the ArtBlocks smart contracts, and save them into a HTML file to run it locally.

probably a more reasonable concern (rather than the on/off chain question) is one of dependencies (some depend on common libraries like p5) and possible incompatible changes to JS and/or browser spec in the future. At which point emulators may need to be created to continue to display this work.

Generally, the burden of maintenance for AB pieces is quite low and archivability quite accessible, relative to many other digital real-time artworks in museums and galleries today.

Agreed. I think the biggest "existential threat" to Art Blocks is that browsers change very quickly and different browser engines do things differently. In that respect, the art is truly just the code and not the visual product the code produces. If Art Blocks pieces are goin to live forever in "live view" it will probably be up to some digital archivists to make that happen.