Just make it work only with your server’s players. That way, if people really really don’t like the feature and quality of life it brings to others, they can roll on the other servers that aren’t high/full population, because then groups would never be automatically filled so the tool would become useless by default.
So now everyone that supports dungeon finder could simply roll on the same servers so that it works, and everyone that wants to keep doing it the vanilla way manually can just keep the other servers alive.
And also, this still keeps things personal and gives repercussions to bad apples who ninja or wipe on purpose, etc, because if enough people block them they will have a hard time getting groups, or their queue times will skyrocket.
A good comprimise is one that neither side is completely happy with, and I think same server only dungeon finder would fit into this category.
Also, to help pay for dungeon finder being implemented, we could sell a one time duel spec option to anyones character, a permanent thing you can pay like 5 bucks for per character that you wish to have duel spec.
Old school players can simply not buy it, and everyone else can spend 5 bucks if they want.