I can agree with this to a certain extent. Monero’s team actually accomplished this incredibly with Tari with “merge mining”, where the proof of work of Monero can be used with Tari with no extra computation necessary. Just submit the same block proof to both blockchains.

There are definitely creative ways we’ve seen that allow for Bitcoin to not lose its value proposition of cornerstone ledger with maximum proof of work. But Bitcoin itself definitely has to be one blockchain, because that’s the definition made in the paper. Chain with longest proof of work, fixed and capped supply of 21M. You can’t have Bitcoin be more than one ledger and preserve that. But Bitcoin’s proof of work could be what secures many additional blockchains.