That's a short-term bump. KU only drew like 25k for Baylor last year. Losing gets old in a hurry. You find out what your base is in a hurry and then watch that erode a bit every year. CU just went through that and found out its base is 35-40k. CSU is probably 15-20k. What I'm getting at is that being in a P5 instead of a G5 isn't some cure-all. Some G5 programs draw a lot of fans even when it's not a "name" opponent coming in and have a base similar to CU's. It is clear that CSU is not one of those fan bases.
What I'm getting at isn't the number of CSU fans that will be in the stadium, it's the number of opposing team fans. I think the number of CSU fans in the seats will increase, but I agree with you, that increase will be temporary rather than permanent in nature.
OTOH, I think the number of opposing team fans in the seats will also increase, but that number will be more permanent. So, assuming that CSU's on-field performance sucked, you'd be looking at something like this:
[TABLE=nohead]
|CSU Fans|Opposition Fans|Total butts in seats
MWC|16,000|1,500|17,500
B12 Year 1|21,000|6,000|27,000
B12 Year 5|17,000|6,000|23,000
[/TABLE]
So, a 55% increase in year 1, but a long term increase of 32%.
Because of a having such a low base, an influx of an additional 4,500 opposition team fans would really move the needle for them - whereas we probably went the other way (removed an average of 4,500 opposition team fans) with the move the Pac12, but given the higher base with which we work, it's not nearly as meaningful.
If their on-field performance were good, I think the absolute numbers would change (say, add 10-15 thousand to the right hand column), but the proportions wouldn't: the increase in home team fans would be matched by a proportional increase in opposing team fans - so in Big 12 year 5, you'd have 27k CSU fans in the stadium, and 11k OU fans.