As his notebooks attest, most of his inventions were in the conception stages, and thus untested. He would be a candidate for the kind of artist you describe in your last clickback as content to leave concepts safely in the speculative imagination. It made me uncomfortable if it's what I think it is. An artist can have periods where nothing seems to be going well. Work is either less than satisfactory or not proceeding.
Ideas are hard to come by.
Commissions fall through. Persistant depressive or negative happenings can erode self-confidence and result in crummy work, which quickens the spiral. I have found it best to crawl into myself, take my own councel and continue on with my greater purpose. The sunny days miraculously seem to return. I know several artists who are building boats.