Significantly diminishing returns in terms of, say, “piano skill per hour invested,” but “dollars earned per piano skill” can be such that “dollars earned per hour invested” has increasing returns, rather than decreasing returns, especially when considering world-class abilities.
If X has significantly diminishing returns at the high end, the pure-Xer will have improved only slightly, at the expense of the twenty other things.
Significantly diminishing returns in terms of, say, “piano skill per hour invested,” but “dollars earned per piano skill” can be such that “dollars earned per hour invested” has increasing returns, rather than decreasing returns, especially when considering world-class abilities.