Eh, I'm pretty sure that I still skip YW on boards with unbuyable banes more often than I skip Hag, though obviously the difference is less stark. I don't think there's a way to separate it out in CR data, so this is more of a gut feeling than anything.
So you think more than half of the time that you skip Young Witch, you don't buy the bane? If you buy the bane more than half the time when you skip YW, then you're skipping both YW AND the bane less than 25% of the time.
Intuitively, it seems to me like it should be easier to trash your way out of Sea Hag than Bad-Bane-Young-Witch.
If you're playing Remake or Steward or Chapel and not buying anything, it doesn't matter all that much that the Curse shows up on top of your deck, because it can't really hurt your buying potential on trashing turns since you're not buying anything anyway. So the Sea Hag attack is no much more effective than the Young Witch attack against that type of strategy (which is the only kind of strategy you really consider skipping the cursers anyway).
The bigger difference I think is that against Sea Hag, you have a lot more
time to trash your way out, since the Sea Hag player isn't going to be buying Provinces or ending the game any time soon. But Young Witch is actually a moderately useful card in a deck. It's usually worth like $1 and some cycling. It's much easier to hit $5 on a YW turn than a Sea Hag turn, and you never get stuck at $2.
A simulator example of how much slower Sea Hag is that Young Witch can be seen by pitting Double YW and Double Hag against Double Jack. Young Witch beats Jack by a 2/1 ratio taking an average of under 21 turns. Sea Hag
loses to Jack by a 2/1 ratio taking an average of over 24 turns! Now Double Jack isn't an engine, but it's the simplest example of trying to deal with curses by trashing, while making a non-terrible deck.
If you're concerned that there's some special Jack-vs-Hag interaction, then you can also try pitting Young Witch and Sea Hag decks against other simple bots. Both beat BM easily, but Young Witch takes 23 turns to end the game while Sea hag takes 27. Versus Chapel, they take 22 and 25 turns, respectively. Versus Steward 22 and 26. Basically against Sea Hag, you have 3 extra turns to work with, and that can make a big difference when trying to build an engine.
EDIT: Oh another funny one: Mass Lighthouse beats both Sea Hag and Sea Hag+Lighthouse (by a wide margin). That's right. Mass Lighthouse... (naturally it loses worse than 3/1 to any other curser besides Sea Hag and Familiar)