I seem to remember Sir Alex refusing to register Manchester United in the FA Cup because there were too many matches. I don’t remember how it was resolved.

A think the single biggest factor is that the rules are different now. There are several examples of rules changes that have come in during the last 50 years that would increase scoring. The backpass rule, 3 league points for a win (it used to be 2), stricter foul rules and substitutions all come to mind.