First, SweetRein wasn't ragging on the concept, just asking people to flesh it out more. Since she wasn't saying it would be significantly detrimental, that's not a strong argument that this would be. Second, yes, I do, because a PvE concept and markedly different game type is much less likely to have detrimental effects than a similar game type competing for the same players.
That's because you're viewing the LoL population as a static, homogeneous pool of LoL players/. In fact, there are many different types of LoL players and not all of them would be playing solo queue 5s if they didn't have Co-op (or Treeline, or Dominion, or Proving Grounds...)--not to mention players who might sign up JUST for this mode. The upshot is that while developing this mode would take a significant toll on Riot, releasing it would not take a significant toll on the current playerbase.