Good question. And I think the biggest factor is one you mentioned, unlimited substitution. Those rules were implemented gradually from 1954 until 1964. There are a lot of factors that favor the 60s for the beginning of the modern era.
1974 is of course, the year that the current Div I/II/II format was started.
One factor that favors the 80s or 90s-the end of the NCAA control over TV and the massive realignment of the early 90s. You could call 1996 the beginning of the hyper-modern era. But that doesn't give you much history.
I did an analysis of poll standings and chose 1968 as the beginning of the "modern era." The biggest reasons for choosing 1968 were not particularly strong-it was the year the AP consistently started ranking the top 20 instead of just the top 10 and it was the first season I clearly remember as I just started following college fb in 67. However, there are some good reasons:
1) 1967 IU, Purdue and Minnesota shared the Big 10. Purdue took over 30 years to win another B10. IU and Minnesota are still waiting. MI or Ohio St. (who last won B10 in 64 and 61 respectively) won at least a piece of the next 15 titles.
2) Texas A&M won the SWC for the first time since 56 and the 2nd time since 1941, following SMU in 66. Texas won the next 6 titles.
3) Integration was seriously under way in public schools, colleges and sports teams, helping SMU win the 66 SWC title. Texas in 69 was the last team w/o African Americans to win a national title.
4) Wyoming went to the Sugar Bowl following 1967.
5) Top 10 coaches poll in 67 included WY #5, IU #6, Oregon St. #8 and Purdue #9. Top 10 in 68 were Ohio St., USC, Penn St., Georgia, Texas, Kansas(the one outlier), Tennessee, Notre Dame, Arkansas and Oklahoma. Prior to 68 you see a lot more of old powers like Minnesota, Missouri and Mississippi.
Any year from 1961-1974 (not 1960 since MN won national title) would be a good pick. Some items of note:
Colts OT win in 58 NFL title game "made" the NFL
AFL founded in 1960
Navy was #2 in 1963
Joe Namath contract in 65 made the AFL and significantly raised the importance of pro careers to college fb players
Dartmouth actually managed to get ranked in 1970-last of the Ivy Leaguers
Miami was ranked #12 in 75, 6 MAC teams in 8 years ranked. There were none for 20 years.
1972 freshman eligibility restored
Vietnam and general increase in education dramatically grew the state universities
Private schools and schools in big cities start declining or dropping fb coincident with the rise of pro fb and increase in size of state universities-Its no coincidence Notre Dame started its streak over Navy in 1964, Texas over Rice in 1966 (until 1993) and Texas over TCU in 1968 (until 1992). The top 10 streaks against one opponent included 8 started from 1958-1969.