If I’m understanding the design correctly, my concern is that the backdoorFee is a way to avoid the set of L2 proposers from censoring. Ideally this results in the L2 proposer for a given L1 block always proposing a block (or set of blocks). However, it doesn’t resolve the ability and economic incentive for a L1 proposer to censor the L2 block, whether it is proposed by the trusted proposer or someone paying the backdoorFee.
Another concern is if a backdoorFee payment is separate from its associated L2 block then a L1 proposer can censor one and not the other. But if they’re combined into a single tx you end up with a potentially relevant overhead in data submitted to the mempool, as well as the block itself in the event that a L1 proposer includes multiple backdoorFee txs for whatever reason. This separation of bid from block contents is essentially what MEV Boost relies solve.
Regardless, this seems like an interesting design, though I’m of the (biased) opinion that if we’re introducing intermediary layers to tx submission for L2s, which seems inevitable given the need to buy fractions of blobs post 4844, we should also get fast confirmations.
At a high level Astria’s design looks quite similar to this in that the shared sequencer proposer set will initially be Proof of Governance and will run a PBS auction for block creation. The core tradeoff seems to be a on the shared sequencer for censorship resistance in exchange for fast confirmations. I’m not convinced you couldn’t have both, though the mechanism design gets rather complicated even without getting into siloed vs shared rollup sequencers.