The social aspect of forming groups has nothing to do with whether you chat with your party members while in the instance. It’s worrying that people seem to genuinely believe this is what is meant when they discuss how Dungeon Finder impacts social experiences in WoW.
While leveling over hours, days, and weeks, it’s not uncommon to run into the same person or people from time to time, be it for group quests or instances. This shared experience over time can lead to a form of social bonding. Heck, I still remember during the original TBC leveling a Priest with a friend of mine on a Warrior. We would often run into this shaman while questing in the open world or in instances, so much that it felt as though we were leveling together, and we became friends (Hey Zarth! Wherever you are now).
With Dungeon Finder, while it’s possible that you’ll run into the same person or people in instance groups, you won’t run into people who aren’t on your server outside of those instance groups, and even if you can befriend them, you cannot cross-realm party or queue, so this is somewhat impactful for some people’s social experience in the game.
It depends on what you mean by communities. If you consider guilds that are focused on Dungeon Leveling, then I can see a compelling case being made. 
Already addressed in the point above.
I haven’t seen this point raised in an anti-RDF context. WotLK Classic is already far easier than WoW Classic or TBC Classic, to the point that it’s a lot closer to Retail, where there’s little fear of dying while pulling lots of stuff.
We can definitely pick who we want in our groups. You can form a 5-person party and queue for random or specific dungeons with Dungeon Finder.
This is quite subjective anyway. Retail, as far as I can tell, is far more popular still than the Classic series, so just because some people who play Classic like to bash on Retail doesn’t really mean it’s in a bad state. Further, while the peak population of WoW may have been during Cataclysm, that doesn’t necessarily mean that something suddenly changed with the game that caused people to leave, but who knows?
I personally think that the Dungeon Finder did and does reduce shared social experiences, which can be seen as a negative, especially for those who may be drawn to this type of game specifically seeking shared social experiences.
Cool post! Glad you’re thinking about these things and sharing them with the community. 