These conclusions are not true. The way we imagine this happening on the Bridge Hub (the “hop chain”) is that no user would ever transact on it; all interactions would be XCM instructions. Of course, XCM execution fees will be in DOT/KSM (on Polkadot, Kusama).
The chains that interact with it (Acala, Statemint, etc.) will need to keep some DOT in their sovereign account on the bridge hub. These can be topped up from each chain’s treasury occasionally.
Of course, you still need to account for the fee, and don’t want to give users unlimited free execution over the bridge. But your runtime should have an idea of the execution costs on the bridge, and charge for it accordingly. I.e., a fee that uses the bridge would have:
- Normal fee for transacting on the source chain (Acala, Statemint, etc.)
- Additional fee for the
BuyExecutioninstruction on the Bridge Hub. This could even go into a small pot on the source chain that gets transferred to the Bridge Hub sovereign account everynblocks.
The source chain can charge fees in whatever it wants, as long as it has a way to price and convert that fee to the DOT/KSM fee it will pay on the Bridge Hub and maintain its sovereign account. (Related: Why not just Statemint needs an AMM but (likely) every parachain and we should do our best to agree on a shared interface)
On a more general note, I think parachain developers need to consider their chains qua sovereign entities that can have and manage their own assets/information and express their own voice within the greater network. A parachain is not just an interface that users pass through but need to own every downstream asset/transact on every other chain – XCM, origins, and autonomous runtime hooks allow chains to abstract this away from users.