When the channel counterparty does it, yes they might be able to steal HTLCs if the victim isn’t willing to fee bump high enough.
A random person indeed can’t steal funds via this attack, but they can grief the victim. They don’t need to risk much in fees either if they already have a low-priority ~1000vB transaction they want to get mined anyway. In fact we’ve already seen a similar kind of accidental pinning in the wild with expired anchor outputs. Are you sure that no one on the network will ever want to grief in this way?
A mining pool may stand to profit nicely if the victim pays 50% more in fees than they normally would. The EV depends on:
- the probability of the pinned transaction confirming before the victim fee bumps enough to escape the pin, and
- the percentage of hash rate controlled by the pool.
By using a keyed anchor, we can limit our exposure to such attacks to just the channel counterparty, instead of allowing anyone on the network to do it.