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.