The <a href="https://en.wikipedia.org/wiki/N!_conjecture">$n!$ conjecture</a> is perhaps another near-miss.  It was posed 25 years ago by Garsia and Haiman, and the ideas in the paper are interesting and non-trivial, but the conjecture itself was probably the most interesting part of the paper, and was a major stimulus for the next ten years until Haiman finally proved it.

Along similar lines, the <a href="https://arxiv.org/pdf/1508.06239">shuffle conjecture</a> was introduced in 2005 and recently proved by Carlsson and Mellit.  The conjecture itself was the most interesting part of the original paper and it stimulated a lot of research.