side-chains is a design that is practically trying to anchor endless competing solana-like networks on bitcoin. the cost to store data for solana is approaching zero. the anchoring is a few bits of hash. swaps are pointless

where will the fee to solve the "security budget problem" come from?

there is no reason to limit the capacity of a "blockchain" that theoretically inherit its security from a hash on bitcoin. the end game is one node that is also the validator, the API provider, and the miner