In general, trying to tailor a cost structure to a specific industry ahead of time is challenging, because you're trying to hit a moving target. Even if it's an airline case, what if you're solving for an airport operator (which operates planes for airlines like Delta), or lessor, or flight simulator manufacturer? The usefulness of starting with variable vs. fixed is that a) the interviewer can follow it easier (as that's probably how they're thinking about it, and how their numbers are structured), and b) having a bucket for variable ensures you can better tie costs to revenues. For the airline example, you'll have a whole series of costs related to food or luggage you might cover in a framework like yours that trys to get them all ahead of time.
Finally, MECE MECE MECE. that's the key for all of this.