Johnny only had six Top ten singles in his career...3 in a row in 1957, one in 1962, one in 1963 and "Too Much, Too Little Too Late" in 1978.
Not only had it been 21 years since his last #1 and 15 years since his last Top 10...it had been 15 years since his last Top 30. And of the 34 singles he released between 1966 and 1978, only three made the Hot 100 at all (at #96, #75, and #54). Did a new generation "discover" Johnny after that #1 duet? No. He released 11 singles in the 14 years following "Too Much"...only three made the Hot 100...at #47, #38, and #81.
All that illustrates why you couldn't just dust off Johnny's version of "Twelfth Of Never" and expect to score (on the charts, not on a date, Landtuna) in 1973. Johnny took that song to #9. Donny Osmond hit #8 with it. His voice changed just in time.