"I’d be like the few older workers I’d dealt with in my career. I felt like they moved too slow, were stuck in their ways, and unable to change - even when faced with evidence to the contrary."
First, keep in mind that many of your co-workers will still feel this way, even if you provide evidence to the contrary.
Second, after you have been around the dharma wheel too many times, it becomes difficult to hop onto the hype-train as quickly as you used to, which is one reason many of your co-workers will feel that way.
The tech industry has never had a career path for technical people other than directly into management or into a pseudo-management "architecture" roles that are neither technical nor management---you don't get many of the advantages of either and what you can do is mostly based on your personal relationships with managers or technical people. As a result, if you remain technical, you may find that your salary and influence stagnate.
Oh, and 40 is still pretty young. I'm 56 and it wasn't until the last 8-10 years that serious burnout set in. (Salary isn't really an important factor in job satisfaction; influence very much is.)
Personally, I've never had to write a letter like yours because I shot the suggestion of a more management-oriented position down in flames, twice. Lucky me! :-)