Thanks. I understand however that if the scammer manages to confirm his "old-state closing" transaction on-chain and the victim doesn't close the channel before the timeod, either because the victim is offline or because the blocks are full and they didn't get space for their own channel update, then the attack could be performed. Or am I understanding something wrong?
You get it right, but it is the same situation as closing LN channel with old transaction state today. If you can reach one confirmation for some old channel state in LN today, you can steal money from people. That attack can be used in LN and in Eltoo. The same is for dust amounts: they are passed as transaction fees and if something goes wrong, then the miner can collect that coins, because no additional output is created for them.
In LN, that problem is solved by creating a penalty transaction that takes all coins from the cheating party. In LN you need one penalty transaction for each old channel state. In Eltoo you will need only one transaction that can be used as a penalty for any previous N transactions, up to some sequence number or something like that, in this way you can discard previous penalty transactions and store only the latest one.
This is the incentive problem I mean, the scammer can even try flooding blocks with transactions himself to prevent the victims from closing successfully.
I guess that if you flood blocks with transactions, then you have to constuct that transactions somehow. And for that, you have to for example move coins inside Eltoo, paying some second layer fees for each transaction. So I guess that if nodes will be cheated by flooding, then fees will rise, maybe first-layer fees or second-layer fees. Now you can lose all of your coins as a punishment, I expect that if second layers like Eltoo will grow, then that will be reduced to half of your coins, fee-based fraction of your coins or something similar to make attacking unprofitable.
But is this enough?
Yes, I think fees will solve that issue, because fees are needed as an incentive and as a protection from flooding. The more flooding will be there, the more fees will be added to each transaction by nodes. So, if some nodes will be scammed by that kind of attack, then I expect that node operators will rise their routing fees, making that attacks less profitable. I can imagine a system where there is no constant fee rate, but where it depends on how well your node behaves and how many transactions you make.
For example: in Phoenix wallet you have some fees set at 0.1%. That means for each 0.01 BTC (one million satoshi) you have to pay one thousand satoshi in fees. So, when you move 1 BTC, it means you have to pay 100k satoshis in fees. On the other hand, you could transfer that amount on-chain and pay for example 500 satoshi when mempool is almost empty, no matter how big that amount will be. So, I guess that different layers will be used for different purposes, you won't have one layer fitting all use cases. The same with LN and Eltoo, I expect some people will prefer taking all coins as a punishment, some people will prefer taking only the smallest allowed fraction of coins as a punishment, and the free market will set that point somewhere in between.