I can personally* confirm that that is exactly how sites like Orbitz and Cheaptickets worked 4+ years ago. Lots of Java and Linux up front. But deep in the back a bunch of 3270 terminal scrapers to mainframes. But that was then. Hopefully they've upgraded those things by now. Despite the principle of "if it ain't broke, don't fix it" there's also the issue of decreasing compatibility and decreasing availability of folks who know how those things work and can maintain them.

(* If you've bought air tickets or made car rentals on either of those sites, at least back during that era, it's almost guaranteed my code touched your transaction at various points in the search and booking lifecycle.)