You brought up the European example, but the European air travel market is very much not like the Regulation Era US market. It’s a mixture of highly competitive discount carriers and legacy national carriers, the latter of which (particularly Air France) have chronic issues with profitability. There’s not much different between the discount market in the US and the one in Europe, and little support for a national carrier in the US (especially given the history of Amtrak when it comes to passenger rail).
You can undo mergers, but going back to a Regulation Era regime would be politically impossible. Mandating a bunch of unprofitable destinations would require major hikes in ticket prices overall for cross-subsidization (very unpopular), heavy subsidizing of those routes (also unpopular, perhaps less so), or even more severe reductions in the quality of air travel.