Vidu basically explained the entire argument. I'll give a personal example of why it applies. Back when I was a wee little sprout, there were the coils. I was 'carried' through t1-4 without really realizing I was being carried. Didn't have much problem with the fights (This was after 2.4 came out, probably a month or two). I still have no idea if I was pulling my weight or not for those, but that's not the point. Then there was t5 and Twintania. The group I was with decided to do it another day. Since I was most likely carried through t1-4, I had no idea there was stuff that actually took a significant amount of effort to clear. And so, I went in t5 with no idea what to do, no practicing my rotation (I used my fire stuff as a blm, but I was far from great), and almost certainly barely meeting the ilvl to get in.
How do you think that went? I can guarantee you this kind of thing WILL happen. I obviously know now what I did then, but there's a constant influx of new players who have not learned that yet. And with all the lvl 50 stuff just being plowed through unsynced, there's even less to teach them that bad play will kill the entire group. Now yes, I COULD always only go in with a premade party. But here's the thing: Vidu was right about the accessibility of content affecting the difficulty. And what's more, there is even a pattern of SE doing just that in the past. Have you not seen the nerfs done to the raids when they were added to the duty finder? They do that entirely BECAUSE the content is added to the duty finder! It's because there is no guarantee of a composition that can even deal with the mechanics. For example, a6s required someone with the ability to silence the first boss at just the right moment, or else it was almost certainly a guaranteed wipe.
I know very well that you're thinking this won't matter for palace of the dead. But I can almost certainly tell you that it will not be as smooth as you think it will be. You won't get very far with a df group without healers. The mobs on the highest floors will murder the squishier classes without a single thought. Or perhaps you think you can clear floor 200 with only 4 machinists? It is this fact that REQUIRES that certain party compositions be excluded from df. Once you start requiring tanks and healers, queue times drastically increase. That kills a lot of the participation right there due to queue times. Add to that the requirement that you can only be on a specific set of floors on a specific class/job (similar to savage raid finder queuing without the job requirement), and the available pool shrinks further. Add to that the penalty of starting back at floor 51 if you wipe, and that MASSIVELY shrinks the available population even more. The community is going to be paranoid about getting matched with bad players, because let's face it, there is always that one person that queues into sophia extreme for the first time with 'duty completion' selected as the purpose, also having a large impact on the number of people who will queue. At this point, queue times are going to be so long that you will probably get a queue for a12s on the NA datacenters faster than you will get a queue for the later floors to pop.
So what can SE do about this? They can remove the penalty for wiping on the later floors. This would certainly boost people queing. They can nerf the mobs so a party of 4 machinists can survive the onslaught. This would ensure most groups could clear it regardless of composition. A few other things and it would be fine for df. BUT WAIT, NOW IT'S NOT THE SAME. THE DF OPTION HAS AFFECTED THE OTHER PEOPLE.