RBY OU worked out, and there was a wide selection of usable Pokemon slightly below standard. Just because it's 4th gen doesn't mean that the number of usable Pokemon needed to make an overall balanced metagame should go up. I personally wouldn't mind if it was an RBYish metagame dominated by a select group of standards but that allowed a fairly healthy selection of substandards to be used competently in unique roles.
Comparing the difference with Groudon versus Armaldo and Tyranitar versus Armaldo isn't really fair. Of course Groudon is "more better" than Armaldo than Tyranitar as a consequence of the fact that Groudon is better than Tyranitar at all, and 2 tiers is bigger than one. A more fair statement would be one comparing Groudon to Raikou (a random old OU Pokemon, though who knows how he'll fare in DP) versus Tyranitar compared to Armaldo. I don't think the degree of separation is really as large as you would think. Outclassing insane numbers of Pokemon isn't a fair argument either as look at the RSE standards and then think of Ledian, Ariados, Ditto, Unown, Luvdisc, Mightyena, etc. NU is going to be massive either way, and I don't think there's a workable model for a standard metagame that keeps more Pokemon above the waterline than below. Even still with ubers, remember in RSE that Pokemon like Exeggutor and Shedinja actually benefitted from being in ubers so they too will contribute to the numbers in this potential metagame.
The real issue is working out what exactly constitutes balance, and how many Pokemon we really need. I think RBY's level of balance (some Pokemon are still blatently better than others, select group of OU, fair selection of sub-OU Pokemon that are still usable in standard) would be a good place to put around the minimum, though I suppose some people don't think RBY is "enough".
Also, about extremely negative comments, you act like the whole thing is a chore, and you act like new people couldn't enjoy such a game. Do you think people really want the metagame dictated to them? I would much rather, as a new user, come into a metagame test and be able to explore rather than come into something already rigidly defined and be told "people better than you decided this is best, and they know because they're so smart - enjoy". You point out that nothing can come in on a Swords Dance Groudon without the correct prediction. Well, tell me, if I had a Choice Band Tyranitar and knew exactly what you were going to do no matter what it was, would you be able to fight me at all? I'd just own you with Stone Edge/Crunch/Focus Punch, and I'd laugh while doing it as you wouldn't have a single Pokemon capable of dealing with me. The only counter to both is prediction, and that might just be the type of game DP is.
If the statement that these kinds of Pokemon don't balance with each other even is true, then well, we'd just have to ban them. The point of a test is to find that out, right?