Bill Lear is rolling over in his grave. To see a company go from a vision and be cutting-edge to becoming phased out is truly a sad day in aviation history. From what I've read, I'm not sure why Lear wouldn't keep up with aviation development. The biggest complaint I've heard is that they didn't ever go with a full stand-up cabin. Gulfstream, Bombardier, Embraer, and Cessna all developed cabins that someone 6'5" or so can walk without hunching over.
That, along with a changing market dynamic, is what killed Lear. The original Lear 23 was somewhat designed off a Swiss fighter at the time. Affordability and high performance is what sold it.
There was nothing like it when it hit the market. There were big gas hogs like the North American Saberliner, and the 4 engine Lockheed Jetstar, (Goldfinger's airplane). But the Lear put small, fast jet aircraft into peoples lives in ways, (cost and size), that the others could not.
Bill Lear was a smart salesman, as well as a brilliant aircraft designer. He, along with Clay Lacy sold A LOT of the original Lear 23's and 24's to high profile entertainers at the time. Frank Sinatra, Danny Kaye, and even Arnold Palmer all bought one.
I think the first one's came with a price tag in the $350,000 to $400,000 range.... A lot of money in 1964. Considering back then you could buy a new Cessna 310 for right around $50K, and play Sky King.
Right around the same time there was a company operating out of Sarasota, Florida called Cavalier Aircraft. They were doing ground up restorations on surplus P-51 Mustangs. (Which were everywhere for cheap back then). You could buy one for right around $75,000, all ready to go. Now a fixer upper will cost you a million + or more.
Then as time marched on, other companies joined the business jet market, size and super luxury came more into play. Along with long range, (which the Lear never really had).
In response to keeping performance alive, Lear even went as far as to hire Neil Armstrong to fly the, "Lear 28 Longhorn Project". He proceeded to set a bunch of time to climb records with it up to 51,000 ft.
But leather, in flight champaign, size, and luxury took over the private jet market. As opposed to small, high performance business aircraft. And sadly the Lear was left behind.