I keep seeing people going on and on about their class being dumbed down, monks are doing the same right now and believe me, these changes don't make things easier. You're actually gonna have a harder time now mastering your rotation NOT easier.
If all the runes are the same, it means bad players will just keep smashing their hardest hitting ability every time they come off cooldown. The current system doesn't let them do this, in fact it rewards them for smashing any button that lights up because they can only exhaust certain rune types as it stands - not all of them.
When 1 rune type is in play, you can't play whack a mole and smack whatever lights up first on your action bar. If you do, you're going to starve yourself of resources and probably be using the wrong spell at the wrong time etc.
In laymen's terms, by giving you the freedom to use any spell you want, it is like removing the stabilisers on a bicycle. That doesn't mean you're going to do well without those stabilisers, or in this case, fixed rune types.