How long will finality take on mainnet?

In the decentralized phase, where many sequencers operate simultaneously, the above makes no sense and the notion of L2 blocks loses meaning

Yes, I had been thinking the same.

We can’t really formally distinguish between “L2 finality degrees”

All finality requires some degree of trust (these are “trust minimized” systems, not “trustless” systems.). Even in e.g. Bitcoin, it’s possible (but unlikely) that the whole community could decide to fork and rewrite history.

I think there’s actually high value even in finality that requires a fairly high degree of trust. For example, Starkware is strongly incentivized to run an honest sequencer because if it came to light that you were reordering/censoring, you would lose a lot of your community.

I’m generally a proponent of Ethereum-level decentralization, but I’d rather delay decentralization of the StarkNet sequencer until there’s a decentralized version that also has fairly fast finality. The UX hit is just too huge otherwise — hour-long finality would be a disaster for many applications.

Second, designing a protocol whose participants are the different sequencers where the goal is to reach a consensus over the next checkpoint… publish the checkpoint on L1

I like this idea.