Well, too bad that we dont have access to the numbers.
IF more than 60-70% of the Powerplayers are doing it exclusively in open - then it could be interesting to make it open only. The other 30-40% might quit PP at all, but probably some of the will switch to open.
We do have the numbers actually since the majority of Powerplay activity is done through the organised community groups and we actually record who is doing what and over 90% of activity is in Open. The main exceptions are the 5C commanders & bots who deliberately use solo & PG to avoid being intercepted and the randoms flying around not knowing what they are doing whose activity is mostly background static.
CMDR Justinian Octavius