AHC: The tradition of presidential (general election [1]) debates begins *and ends* in 1960. That is certainly the way it looked in 1964, 1968, and 1972...
And no, the "equal time" rule (which made debates impractical by requiring that minor party candidates be included) and the FCC's eventual decision to get around it by saying the networks could cover debates as a "news event" are not the central issue here. Congress could always suspend the rule, and if it refused to do so, everyone would know that meant that the candidate whose party had a majority in Congress didn't want to debate. The real key to the revival of debates in 1976 was that you had an incumbent president who was way behind in the polls and therefore--unlike LBJ in 1964 and candidate Nixon in 1968 and President Nixon in 1972--wanted a debate and a challenger who was sure he could outperform the president. Under other circumstances, one could easily see no debates in 1976. And even after the debates in 1976, it was not clear that the tradition of debates would continue. Even in 1980, Carter skipped the first debate because of the inclusion of John Anderson. What if the League of Women Voters had also insisted on including Anderson in the second debate and Carter had skipped again? And what if in 1984 Reagan had decided that with his lead over Mondale, he had nothing to gain and potentially much to lose from a debate?
The fact is, the more debates there were, the more "unthinkable" it became for front-runners to dodge a debate. The fewer debates, the more "thinkable" dodging them would become.
[1] For primaries, there was of course the Dewey-Stassen one-issue debate ("should the Communist Party be outlawed?") of 1948
https://en.wikipedia.org/wiki/Dewey–Stassen_debate (the last Republican primary debate until 1980) and the Stevenson-Kefauver debate of 1956.