> You may be on a team that has decided "developer burnout"
No. Burnout is a work life balance thing more than a "doing work at work that is related to the business" thing. Fun can be had outside of work hours, just as everyone else who is employed handles it. You can give people meaningful work they're interested in and still keep it related to the actual business.
> Which is a problem in and of itself.
Absolutely not. If you don't have a near infinity list, then that means you don't have a roadmap. Why aren't you thinking about the future?
> Call it "skill investment time."
Yes, and this can always be done in the context of the work. If it's not related to the work, or the business, then it's not an investment for the people paying you.