I had the same gut reaction. It may at least be possible, if extremely difficult, to get past the need for good repute ... LN penalty not ideal here, clearly.

The attack you describe was called "forced expiration spam" in the original LN paper. I usually call it an "expiration flood". Other people have other names for it. See

Bitcoin Optech

Expiration floods

Expiration floods occur when many timelock-contingent payments need to be settled onchain within a limited period of time. If not all of the settle...

I don't think LN-penalty is the main problem here, as the most powerful form of the attack starts by filling channels with as many expiring HTLCs as possible, which is a problem for any channel design. The only solutions I'm aware of are (1) temporary dynamic block size increases, which only increase the cost of the attack by a small multiple, (2) some form of time stop, although that increases the risk of capital losses from the time value of money, and (3) various bond designs, although they upfront accept losses to the time value of money. Of those options, I think bonds may be under explored but I also think that the main downsides of time stop may be almost entirely mitigated by John Law's hierarchical channel factories design, which would involve channels being opened by three parties, with two preferred partners being able to continue exchanging funds in the channel even if the third counterparty initiated a force close that was taking forever due to a time stop.