I think if they can figure out a way to offer the 4NF but target it toward leisure travelers rather than road warrior consultants, it would be sustainable.
Maybe impose a weekend requirement or something, or cap it to some times a year.
It's a key benefit of the card, but we only use it a few times a year, when we're on holiday.
If it flushes out some cardholders getting 10k a year out of this benefit, I would not be overly sympathetic.