Here is the real reason why Mages are getting gutted:

Because Flashy, #OMGDATOUTPLAY!, fast paced energy-based AD assassins look way cooler in tournaments, which gives more viewers, which gives more money.

Aka., the only thing this game is about nowadays. How can we please the largest possible number of people who started watching last week, even if that means destroying the game for people who played it for years.