there's no other way it profit from such an attack
This statement isn't factually correct. There may be many additional incentives.
For example, a bank could profit financially from destroying Bitcoin's image because people would return to the traditional financial system. At $84,349.58
for a one-day 51% attack*, I'd say it's chump change for financial institutions; add in a couple of double-spends, and you'll not only fuck Bitcoin permanently, but even profit!* Cost amortized over a year of operation. (Caveat will be removed for conciseness.)
You can't add in a couple of double-spends, you can only pull of one double-spend before everyone on the network realises there is something very wrong, at which point everybody will stop accepting bitcoins until the attack is over. Once the attack is over, the network will resume functioning as normal. A one-day 51% attack doesn't fuck Bitcoin permanently, it only fucks Bitcoin for one day.
One double-spend can contain multiple double-spent transactions, fucking over multiple merchants... I don't see your point.
It does fuck Bitcoin permanently. People would lose trust in Bitcoin, and I doubt many would hang around.
Edit: Because the attacker is only mining on top of their own chain, mining profitability would plummet and miners would be forced to leave the game, initiating a vicious cycle where the attack becomes cheaper and cheaper.
One of my favorite posters, SgtSpike, sums this up nicely:
I, for one, would leave Bitcoins altogether if someone was trying to forceably control the Bitcoin network with > 51% of the hashing power. And I know I'm not the only one...