Type certificates for commercial airliners should expire after 30 years. After that date, allow any existing aircraft to continue operating but don't allow manufacturing of any new ones. The manufacturer should have to recertify and prove that the design fully complies with all current rules.

The original Boeing 737 design only took 4 years so that would give them plenty of time to work through any necessary design changes. And if pilots need to obtain a new type rating then so be it.

The type certificate was for pilots

> So, many airlines (eg southwest) will indeed only use one type

And yet many airlines fly a very high amount of planes of many types, of multiple brands. Southwest is a consumer darling and punches above its weight but it is an outlier.

Well, Southwest was a successful well-run regional carrier that grew as large as the big mega-airlines, while trying to keep running the business the same way.

There are still 20 smaller regional airlines in the U.S. and hundreds around the world. The vast majority of those fly just one or two types of planes.

> And yet many airlines fly a very high amount of planes of many types, of multiple brands

That's simply untrue. Most airlines use 2, 3 types of planes at most. I'm not referring to the niche 5 percent of plane types but the overwhelming majority.