This is true here and now, but in the future, it can be changed. Because each time when something is sent as fees, the new coinbase transaction starts a new "chain of signatures". Which means, if you want to migrate transactions from one chain to another, then you have to follow signatures only to the nearest coinbase transaction. Because then, it could be made from original fees, or from any other fees, which means, it may be possible in the future to discard the chain of signatures, after the nearest coinbase transaction, if Ordinals or other abuse will force full node operators to do so, and to optimize some things in some future model (for example UTXO-based model).
Are you sure about this? I think the mining nodes would still have to keep an entire transaction history. Nodes that sync from the genesis block need every block to calculate/verify the hashes of every block leading up to the latest block. This requires at least some nodes to have every single block. This means that there can never be zero seeders for the entire bitcoin blockchain. Please correct me if I'm wrong.
P.S. I looked into commitment signatures and I think this would be a great replacement for ordinals in the application that I plan on building.