Your question makes the assumption that energy density is the only factor to transition from one fuel to another. In fact the world currently uses many different fuels rather than one exclusively. According to the link, hydrogen is the most dense, so we should be pursuing a hydrogen economy, although the link omits nuclear. Following your logic nuclear fusion which is the fusing of hydrogen atoms (currently unavailable and always at a promise of only a decade away) would be the most energy dense source, not fission.
The huge factor you have omitted is safety, which unfortunately is usually trumped by economics. Power generation by fission relies on a thread of unsafe practices from mining, transportation, enrichment, generation and waste disposal, none of which have been demonstrated safely. Fission also does not stack up economically.
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