> The data has to be sourced from somewhere, which would be the airlines.

Such nonsense.

Pretty much all airlines don't own/run the source of truth for their flights and haven't for decades. They outsource that to GDS[1] such as Sabre and Amadeus. Their backends are not fragile.

You just have to pay for every query thus the caching.

[1] https://en.wikipedia.org/wiki/Global_distribution_system