How about:
1) start building your own set of data to complement the bought data. Every time there is an inaccuracy, approach the airline company and start using their data directly. Later on, you can even become one of the companies who provide data to others. When there is a problem ("the data companies are not doing their job") there is an opportunity.
2) change the pricing model to charge big users more. They should be your bread and butter, because they get paid to use your app. You are just a small additional cost to them. You can even lower the initial price tag and charge more for enterprise usage... Since you are better than others (see 1.) the enterprise users will be happy to pay more.
3) failing that, sell to an entrepreneur who will continue. I am sure there are many interested persons and your users would be better served if someone picked up where you left off. Hell, I am interested... :)
I hear you on the time point. Thing is, it's hard to get to that place where you have an offer on the table. Starting over would have a time cost, so over the long term, you might've been able to buy yourself more time with the time you'd already invested.
Of course, that also depends on the amount of the offer.
Have you considered selling it to another entrepreneur to carry on instead of shutting down completely?
In my experience, consumer mobile is pretty much tapped out - it's very difficult to make a living as an independent app developer. But the enterprise market is still booming. Now that the giant consumer applications (WhatsApp, Instagram, Uber, Instacart, etc.) have been found, there's a long tail of smaller niches that'll pay good money for specialized services.
Further, the data sometimes is bad and causes inaccuracy issues for customers without him knowing, this causes lots of complaints that he has no control over, something that isn't feasible when charging money to commercial users. Not just that, but the price of data is ever increasing, free data is becoming restricted and he needs to restructure as data suppliers go out of business or restructure their API.
I mean there are obviously solutions, like generate his own data etc. But it really looks like he's got other opportunities to earn a living and this isn't the lowest hanging fruit for him.
What I DO find strange is why he's not selling it. I just can't imagine not at least one of the company's he's buying data from wouldn't buy this at a discounted price if need be. Some money is better than nothing.