I think you tend to take the narrow view, Erik. While there are perfectly good reasons for using new MCU's, if you have an application that's been working fine for 25 years and you're asked to add a function, one option certainly should be augmenting the code. Along with that, should be the option of swapping the MCU to one that can execute instructions faste without messing up the bus cycle timing to such extent that the board no longer works.

If you're producing 1000 units per week, yes, you can justify using the latest technology, but if you're tweaking a half-dozen boards, boards that have been happily doing their job since the '80's, and just want to add a bell or whistle, wouldn't that make more sense done on the existing hardware?

RE