Do academics get bored of research?

What helps, I think, is that "research" is not a single thing but a complex of many different activities. If I am feeling burned out on paper-writing, perhaps I find my joy in coding or mathematics or sketching new project ideas. Likewise when I am happy about papers but feeling burned out on something else. At a larger scale, even a "unified" line of research has many distinct facets that may feel quite different when one is actually closely engaged with it. Thus, I see no difficulty in the idea of remaining interested and engaged for a long period of time. I have been so for nearly 20 years (counting undergraduate research work as well) and I see no reason to expect my interest to fail any time soon. Funding, of course, is an entirely different story.

My spouse almost never gets tired of research. This is a person who loves collaborating with people, helping students, tinkering with equipment, tinkering with data analysis programs, analyzing data, writing papers, reviewing papers, editing papers, planning the next experiment. My son, on the other hand, is cut from different cloth. As my spouse would say, this is a guy who is more of a tool user than a tool maker.

I heard an interview with a surgeon on the radio once. She said, "Don't become a surgeon unless you feel that you can't do anything else." Meaning, unless there's nothing else that would satisfy you. That's the way my spouse is. I don't think anything else would be anywhere near as interesting or satisfying for this person.

My spouse did change sub-fields about halfway through, feeling that the first sub-field was well understood and rather saturated, and wanting more of a challenge. It was a good change.

There cannot be a single answer because people are different.

I'm on a first name basis with a nobel laureate in his seventies who abandoned all academic work immediately after retirement; he says that he could no longer bear the politics, jealousy, intrigue in academia.

My PhD advisor, on the other hand, just turned 92 and still goes to university two or three days per week. He has macular degeneration, but uses a 27" display to keep up with the literature in his field. He's aware that some of the younger profs at the department think that he's just an old fool, but he doesn't care.