This topic has been discussed to death, but sure, I’ll bite.
First of all, I strongly disagree that the LFG tool has been a failure. I personally find it convenient. You can see what a particular group’s composition is, which gives you insight into what your competition on loot will be, who is in the run, and more. The LFG tool simplifies what we had before, which was LFG spam, while adding additional info. Now of course, it does have flaws, such as being buggy sometimes, and the request to join feature, the existence of which still confuses and annoys me. I used the LFG tool frequently to find heroics up until about 2 months ago when I finished gearing all my characters, and I still use it frequently to find people for H+ and pug raids and I am generally a fan of it.
Presumably, you’re saying it is a failure because you are finding it hard to find or get into groups, and maybe some clarification on your end would be good here - what sort of content are you applying for that you are getting declined for? And what sort of effort are you putting in to actually complete the content you’re looking to complete? Are you trying to form your own groups? Are you just giving up because one person asked you for your gearscore and your number wasn’t satisfactory for them? Because I can guarantee you that there are many players out there who don’t care much about gearscore.
As we see more and more people kicking back those that don’t hit their particular “gear score” wants. Many get left to the way side. We are also seeing difficulties in finding dungeons. This includes all types of dungeons. No, we wish to not carry people of course.
Again, kind of goes back to lack of clarity on what content you are talking about. People will naturally look at applicants for Titan Rune (H+) dungeons with a bit more scrutiny, because there is a lot of potential to fail and waste time in H+. Now, what you’re suggesting is that instead of a system where players attempt to form a capable group out of the available strangers perusing the LFG tool/chat, we should instead implement a system where I queue up and am then forced to play with random strangers to attempt, and likely fail, to complete the difficult H+ content. Hard pass, thanks.
I think the bigger issue here is that H+ dungeons are difficult enough to warrant the aforementioned scrutiny while simultaneously giving very weak loot rewards. Casual players may look to H+ as the next step to progress their characters after they are done with heroics. And yet, the rewards from Naxx10 are much better than the ones from H+, despite H+ being similarly difficult, if not more difficult, and despite H+ being way more tedious.
For heroics (not H+) the debate just goes back to the one that’s been beaten to death. Yes, it would be more “convenient” at the cost of losing the opportunity to make social connections, and a rupture in the immersion of a game - you now just sit and Dalaran and get TP’d to the dungeon - no longer the “World” of Warcraft. But then again, heroics are mostly pointless now, as Naxx10 is so easy and gives insanely good gear compared to heroics. Meanwhile you can buy enough crafted or BoE gear that you really don’t even need to step foot into a heroic to be geared enough for Naxx10. Normal heroics are basically not needed anymore except for some niche pieces of loot. And maybe that’s something that should be changed, rather than adding RDF.
You do bring up a good point which is that lower level dungeons are pretty tough to find groups for, which I do think is disappointing. Although, rather than lack of RDF, I see a big reason for this being the fact that they simply aren’t good experience/hr compared to efficient questing, which is now more popular than ever due to a particular (for-pay 
) addon. As a pretty experienced player, I typically avoid doing dungeons when leveling up because I just know the average expected exp/hr I’m going to get from dungeons makes them not worth it, and with the implementation of heirloom gear and weapons, there’s really not a lot of incentive to do dungeons from a gearing perspective. For the slots not covered by heirlooms, I can just buy better gear from the auction house usually. I do think the team could look at buffing rewards from dungeon quests, or maybe give additional experience rewards for killing dungeon bosses. I also really hate heirlooms in general as a concept, but I guess that’s a topic for another day.