Paterno had one losing season (1988, 5-6) in the 27 years as head coach before joining the B1G. He had four losing seasons in a five-year period after joining the B1G. While Paterno did quite well initially in the B1G because of the increase in TV exposure and the resulting recruiting bump, it wasn't all that long until they started their slide into relative mediocrity due to the punishment of playing in a physical conference like the B1G...
Paterno's overall record at Penn State before joining B1G: 247-67-3 (0.784)
Paterno's overall record at Penn State after joining B1G: 154-68-0 (0.694)
That's a huge 0.090 dropoff.
Yes, that's a drop-off, and a significant one. But, it's not a 100% apple-to 100% apple comparison. More like 100% apple to 90% apple. A few more years to one side, weaker schedules in the 1966-1975 era.
If you do a 100% apple-to 100% apple comparison (same # of years), there is no drop-off for PSU's first 10 B1G years. And as noted above, PSU's SOS was nearly exactly the same in 1983-1992 to 1993-2002.
Let's just get to it. The hypothesis of many here is "Penn State football would not have been as successful in the 1970s and 1980s had they been in the B1G vs. being an independent." Well, it's all hypothesis, can't prove it one way or the other.
I grew up a Michigan fan, in Detroit, in the 1980s. I liked B1G football well before I went to Penn State. I like college football history. One truth I know in retrospect: Although B1G football was very good in the 1950s and 1960s, in the 1970s and 1980s it was neither very good nor deep. It just wasn't.
7 of the 10 B1G teams had a losing aggregate record from 1966-1992. The B1G Rose Bowl representatives went 7-20 in that same span. That despite having a higher AP Poll ranking in 18 of those 27 Rose Bowl games. The B1G went 23-30 in non-Rose Bowl games in that same era.
If you put the late 1970s and 1980s Penn State teams in the B1G, IMO they would have done just fine, they would not have fallen off. I can't "prove" that, but I do 100% believe that. I wasn't a PSU fan at the time, I don't have a bias in that belief. I simply think my eyes plus the numbers bear that out. And, as the 1993-2002 vs. 1983-1992 comparison shows, PSU kept things going nearly the same upon joining the B1G (if one wants to say I'm "biased", I admittedly was a PSU student/fan for a portion of the 1993-2002 era).