This thread is giving me déjà vu
As @Xeux already pointed out, PLEX is more profitable to CCP than actual subs. Also, PLEX helps regulate the economy by serving as an ISK sink. The fact that someone paid ISK for PLEX does not negate the fact that the PLEX was ultimately bought by someone else with $$$. Can you imagine what it would be like in real life if you bought a gift card (ignore the fact that few people use gift cards anymore) for a subscription to give to someone, but they had limited access to that subscription just because they weren’t the ones to buy the gift card? It’s the same BS with what you’re proposing.
With all due respect, what you’re proposing is an impractical solution to a non-existent problem. Better formulate the actual problems that need to be solved, and then identify more direct and simpler solutions.