Oil companies? Airports? Servicing companies? Frequent flier and credit card programs tied to the airlines?

Airlines themselves are perfectly fine. It's relatively hard to break into as a disruptor, but the employees of the established players are mostly well-paid. The executives are even better-paid. The investors can earn steady, predictable, positive returns as long as the stock is reasonably priced and doesn't make assumptions about impossible levels of future growth. Airlines are not super-profitable, but with sufficient volume, you don't need high margins, and they're at least profitable as long as we don't have 9/11 or Covid happening.

I'm not personally knowledgeable about this or anything, so look into it yourself, but just picking a random really big one, United Airlines, it looks like they consistently beat the S&P 500 for most of the past 20 years, until Covid, and the past four years since then have been pretty bad.

They are profitable regardless in the face of 9/11 and COVID because we will bail them out.

What they don't seem to be able to weather is significant changes in the structure of pension obligations, and dramatic fatal crashes.

There is also an impending shift into electric short-haul flights that is going to cause some changes to the business model, with whoever can't adapt exiting those markets.

During the post-2001 crash in airline passenger demand US airlines lost more money in that one slump than all profits from all US airlines, combined, from the Wright Brothers to September 11th 2001.

That was why you had such rapid consolidation, as America West became US Airways became American, United bought Continental, and Delta bought Northwest, to remove so much competition that they could be profitable again.

Wow, do you have a source for this?

https://transtats.bts.gov/Data_Elements_Financial.aspx shows 2001-2005 net income losses were 57 billion dollars, then a brief profitability in 2006-7 but the losses of 2008 and 2009 wiped that all out again (in net income terms).

I read that thing about the integral of all profits back to the Wright Brothers back around in 2010, and I can't find a good source on aviation profitability in the 1970's right now.