RAID is setting higher and higher requirements to book champions - the recently introduced Vildrax requires a massive 17 Epic books to max out!
That's the same thing as nerfing players' rewards - just a more indirect way of doing it to try to avoid direct player anger, which would have been more loud if the rewards were nerfed directly.
It is a smarter strategy by Plarium allowing increase of items over time on one side but a much larger increase of costs on the other side, so those players who are not paying attention think they are getting more when in reality they are getting less.
