Note that I'm not an expert, so I can be wrong here.
Let us assume a scenario some time in the future where Lightning is practical and widely adopted. There are many payment channels open at any given time, as well as efforts to keep those channels decentralized. Now the latter might be hard, because a few highly connected nodes might appear.
Now, consider that one day, some actors controlling many channels decide to suddenly start committing older channel states (those that favor them). Think somewhere in the order of tens thousands. Now everyone on the other end of those channels notices and they broadcast their own states, but the blocks fill up and only a fraction of them actually gets through. When transactions don't come through in time, the attackers profit. This might start a frenzy where people close their channels in the fear of losing coins. The effect can be amplified if collaborating miners start mining empty blocks.
- Broadcast old channel states
- Keep blocks full until timelock runs out
Please tell me that I'm wrong and that Lightning is more secure than I thought.