A key difference, however, is that the 737's evolutionary changes came only with each new iteration of airframe. If you bought the -200 model, you were stuck with -200s. If you wanted to udgrade to -600s, you had to go out and buy some -600s.
Which I've always found slightly curious, given that there have only been ... we'll politely say 'modest'... improvements made to the airframe itself, the vast majority of which could have easily been done in 1967. Any improvements in performance or efficiency have come from more reliable systems, better avionics, and better engines. But you still had to buy new copies of basically the same old airframe if you wanted to get the new goodies.
But you'll find precious few examples of an aircraft series 'evolving' significantly over a long period of time without needing to buy new airframes. They do exist (CH-53 to Yasur-2000, or A-4B to TA-4SU for example), but it's certainly not 'the done thing'.
As Mr Pencil has mentioned, the Super Hornet is an attempt to do that, but with a significantly different agenda. The Super Hornet was not designed to be the premiere combat aircraft of the 21st century. It was designed to be cheap. Cheap enough to develop so that it didn't get cancelled outright (unlike the A-12), and cheap enough to buy that the USN could replace their airframes and still keep all of their big deck carriers (unlike the F-22). One way to do that was to deliver them with no bells or whistles, and off-the-shelf systems from the legacy Hornets. The 'spiral upgrade' program was to bring the aircraft up to the required standard once they actually got the airframes past the bureaucrats and politicians.
The F-35 front-loaded most of that development effort and cost. And so it seems to me to be almost a direct result, that two of the biggest failings of the F-35 are systems development difficulties (time and cost), and sticker shock. Following the Super Hornet method, and delivering aircraft earlier on, with an interim equipment fit, could possibly have taken the edge off of both of those problems.
Of course, the risk with that strategy is what we saw happen to the Tornado ADV. Flew around with lead ballast instead of a radar (literally) for years, and remained behind the technology curve (for various reasons) for most of the next 3 decades. Although, on second thought, I'll just join in the dogpile against the English and blame it on their bizzare love/hate relationship with all things electrical.