Based on what you said, I guess I can get behind Keldeo dropping, but there's still no way Latios should drop. I think that you,
alexwolf, are overstating the impact of these new Megas on Latios. Just because Latios now has actual offensive checks that aren't complete crap/unreliable/situational/whatever else doesn't necessarily mean it should drop a rank. Many of the positives of Latios still remain a factor from XY, and the fact is that it's anything but helpless against these mons, as it can still do severe damage. As in, they're not coming in for free (Diancie, Metagross, Altaria), meaning that Latios can just switch out and have done its job, which would be punching a hole in the opposing team or getting off a Defog. Every team should be carrying answers to these Pokemon either way. Furthermore, while the faster Pokemon are a concern, they only serve to bolster the effectiveness of the Scarf set and aren't nearly as common as Megas like Gallade, Salamence, Metagross, and Sableye. Finally, Latios was never really the balance hazard remover of choice anyways, so I'm not sure what you're alluding to when saying it faces competition with Mega Sableye, which is btw far more effective on bulkier/stall playstyles, which is never where Latios was used. Defog Latios is a hazard remover for heavy offense, bulky offense, and offensively minded balance. And this is a role it faces almost no competition for besides Latias, which is significantly less common than Latios for a reason.
I didn't actively play much during late XY OU when Latios hit S, but I'll try to explain as I understand from following threads.
I think the problem is less that Latios got viable offensive checks, and more that those checks don't have to be put on specifically to deal with Latios, whether they deal with other things and Latios is collateral, or they're new mons Latios can't keep up with.
Weavile, for example, improved quite a bit between KO + Icicle Crash compatibility and the need for its Ice Priority (admittedly shared with Mamoswine) or speed against new threats like Mega Mence, Mega Scep, or Landorus. Latios doesn't like added Weavile presence, since he's outsped without Scarf and can't switch into anything except Low Kick regardless. Or Empoleon, who is one of the few answers to non LK Greninja and laughs at non T-Bolt Latios
Latios is a bit harder to fit into a team because there's several more Pokemon that can handle it while still concerning other members of a team, which means he needs much more multi-purpose teammates to function. To bring in a somewhat extreme comparison of what I mean, let's take Mega Lucario.
One thing that made Lucario so dangerous is that because of its insane power on either set, it had very few viable checks. So all the team required was something tailored to remove that set's checks and Luc could muscle through everything else.
Latios requires team support now for a greater number of threats it can't handle alone like Mega Sab, Mega Gross, Mega Mence (as long as it's here), as well as old problems that are getting more common/powerful and thus harder to deal with, like Greninja or Weavile. It's job isn't as easy to do because more mons can deal with its coverage combos, so it's not quite as easy to pressure the opponent for a Defog turn against offense, and mons like Sableye or the occasional Altaria (I've seen a couple at least, whether or not they're good) give Stall more answers to it to either keep their hazards up or deal with it hassle free and re-lay them.
It also just doesn't like the Speed creeping, since it loses more ties than it wins assuming both mons are healthy and 1v1. This also assumes no hazards, since Latios' Defogging would remove it from both sides.
- Mega Metagross shrugs off most of its moves, especially if it dropped from DM, and can set up Agility or just go for the 2HKO (small OHKO chance) with Meteor Mash
- Mega Diancie can OHKO with Moonblast, but none of Latios' can OHKO back on a negative nature
- Mega Gallade has a tiny chance to survive LO Draco Meteor, but Latios never survives Unboosted Mega's KO. So even if it wins the speed tie, there's still a 70% chance it'll lose
Gallade is the shakiest since he has to already be Mega to speed tie, but Metagross and Diancie have enough of a margin (Diancie taking at most 61% from Psyshock with Mega's reduced defenses) that they can afford to take the hit if they aren't Mega'd and still come out on top. If they have evolved, they have the advantage in a Speed tie with Latios while presenting enough of a threat to dent the switch-in to their move.
A speed tie, while still a toss up, is more likely to be a better gamble for the opponent than Latios. This is mainly looking at the new Megas in Latios' speed tier, never mind either resurging old threats or new Megas that outright outspeed.
