How would this help anything at all? Kaz druid who runs just kaz+onyxia wouldn't be affected, and it'd let decks who use kaz with dragons to just jam him on 8 anyway and start the stupidity just as early. Maybe the solution is to just stop printing absolutely stupid cards which are a self-containing win condition, and iunno, letting people do anything they want because they have 20 mana on turn 10.
It would help (in some ways) because running only kaz and onyxia is not the optimal version of the deck. Being able to run Naga Giants with Elune is a huge upside, when drawing through your entire deck quickly and efficiently, thus making Kaz's condition unimportant, isn't hard.
HOWEVER, it would eliminate some of the nuance around these cards - especially the Highlander ones. I can't tell you how many times since LOE launched that I was running a highlander deck, then idiotically used some spell that would shuffle multiple copies of something into my deck, only to realize I had neutered Reno. (Along those same longs, there would be no room for Albatross tech etc)