While you are correct, we can the low the impact on social interactions for a dungeon finder thats realm wide only. If the main argument against the feature is the social aspect, introducing it as a realm wide version should be alright. This doesnt solve the problem for dead realms though, but Blizzards idea for a tool doesnt fix any aspect that RDF fixes.
People are different. Some people go the the soccer field just to play some games without hanging out the the other players after the game. The only come to play a match, thats it. Its the same for wow. Some just want to run a dungeon because its fun and dont look for a chatting platform.
In my opinion, anyone should have access to casual content like battlegrounds and dungeons without having to be in a special social community. As said, there are many players who just want to play a good game and dont want to be forced to rely on social communities to play for one or two hours a day. If the more challenging content like finishing raids and climb in arena requires a group of players that know each other and improves strategies together, thats great, however not all endgame content should be gated by the need of social bounds.
I actually made a topic about this:
I would argue the other way around. While wotlk wasnt designed to have RDF, it was introduced because it fit in so well and solved many problems. Wotlk was not designed around the RDF, but the RDF was designed around wotlk. The lack of content even proves how well the RDF filled the gap by enabling players to play their alt chars casually and still make progress.
We could use that the same argument against battlegrounds or any teleports like summoning stones, warlock ports or even hearthstones. However, while its arguably not the best way to immerse players by allowing teleportation, it is a necessary evil for group activities. Imagine no teleports available at all, like no summoning stones and a whole raid would have to travel to a location because of the immersion. It makes sense when it comes to the immersion-argument, but is not more valid than the teleport of a RDF.
The dungeons in wotlk are designed to have much less trash and are finished much faster than any dungeons before. With this in mind it doesnt make sense to make players travel for a longer period, than they actually play the dungeon (this is an exaggeration, but I think you get the point).
As said, it would be the very same change of environment if you get a port from the summoning stone or get teleported by a warlock. The teleporting isnt tied to the RDF, it just causes 5 players to get teleported instead oft 3 players. The addition to teleport the players back to where they were was included, so leveling players had no issues to continue their journey.