You bring up a good point in that there really isn't a distinction between P5 and G5 anymore. They are likely to be firmly entrenched as the "tallest midget" conference; particularly when the poach the top teams out of the current tallest midget conferences of the AAC/MWC. Then those Conferences will poach the top teams from CUSA, and then CUSA will poach the top teams from the Sunbelt, who will poach the top teams from the FCS.
As far as falling off like the Big East, it is worth noting that they didn't lose their BCS status and rebrand as the AAC until the final original football member had been poached. At that point they were down to zero (0) members that were BCS when the Big 12 was formed, down from 8.
Interesting point... I decided to look up recruiting rankings for two Big East members, Pitt and Syracuse. Picked 2001 as a start point (due to availability of rankings on 247) and 2011 as an end point (when Pitt and Cuse announced they were leaving for the ACC).
I was thinking that perhaps as the Big East lost its strength/respect during realignment, that they would lose their recruiting ability.

Syracuse with outliers in 2001 (29th) and 2009 (92nd), but classes ranged in the general 55-60 range through most of that time.
Pitt with outliers in 2006 (16th) and 2011 (62nd), but generally much stronger. Their variance is higher, but there's no trendline I can discern.
I don't see a trend either way, supporting your point that the Big East teams continued to be who they were even as the conference was deeply injured by the realignment in 2005. There was no sustained drop in recruiting for either team from 2005->2011 when they announced they were leaving the Big East.
Granted, I don't know if this was enough of a sample size either way... More about the length of time, rather than the number of teams I looked at... I would expect the recruiting dropoff to be a decade+ phenomenon, and we didn't have that much time in this case.