Anything can get boring after doing it for a while. The Witcher 3 game play was just as boring as WoW after 200+ hours spent in the game. It is all about if you personally feel it is enjoyable or not and not about the system being used.
People also signed up for change even 15 years ago. The full game wasn't even released at Vanilla launch as many systems were added in post release patches. Not to mention Everquest changed things and was the reigning king prior to WoW. The genre of MMORPG 15 years ago was one of change and iteration just as much as it is now. Things just change over 15 years.