There is indeed harm done. The harm is known as community fracturing.
What this means is by adding a new permanent game mode will take away from ques of other modes. Now, for the more popular regions such as NA this isn't a huge deal. Maybe an increase of a minute at the very most, like when URF was actually out last year. However the issue isn't limited to these regions as League is a global game. Less active regions (like Brazil iirc) already have low-peak periods that interfere with ques like dominion and 3v3 and have needed to shut-down these ques simply due to the horrendous que times (because there is no one quieing).
This is happening without a new mode. Now, the simplest thing to do would be to simply only add URF to the more active regions that can support it. Good idea.....for the people in those regions. Meanwhile you're screwing over everyone is the less active regions and you get major complaint on exclusivity.
Dominion was made before League was a major global presence and ARAM already had a significant slice of the community in custom lobbies by itself so adding them was cake at the time without having these issues. Now however, that is no longer the case.
tl;dr The issue isn't that simple.
Also: Meta stagnation. Trust me you don't want URF permanent.