Tech's absence from the SEC had nothing to do with its fortunes after that.
Tech's problems were, pure and simple, a lack of a good head coach to replace Dodd.
When Tech was winning big, there were no tickets to be found anywhere. The stadium was a perpetual sell-out. When Tech started losing, there had to be a scapegoat, and, erroneously, it was attributed to leaving the SEC.
Had Tech guessed right on the successor to Dodd and kept winning, the stands would be full to this day.
When you have three coaches in a row to produce winning teams, the fans become accustomed to winning and will not accept losing. Therefore, the older crowds will not attend as long as the program is not consistently winning.
When Tech starts winning consistently again, you will not be able to find a ticket to the games. It is as old as history, build a winning team, and the fans will fill up the stadium, even if you are playing in the Little Leagues.