I never bought anything yet in the Play Store as an Android user, but I never knew that GP Credits exists. When I tend to test an app in Play Store, I tend to download the apk first but somehow I never had the thought of really buying it.

Every closed digital stores could already have its own currency anyway, like for Steam that has Steam Credits for buying games. In farther future, I believe that these popular digital stores would utilize blockchain at least. Even though from the articles that I had read that Gabe is unlikely to put cryptos directly in Steam given the instability in price for it to be a mediums of every accounts in the said platform.