I don't agree with Mega Latias dropping to B+ at all, so I'm going to give my two cents on the matter.
The fact that Mega Latias struggles with Clefable and getting twaved is nothing new. Why do people expect it to be able to beat Clef? If you're using Mega Latias, you need proper support for it, just like any other mega - you can't expect it to break everything. Saying that it struggles with Clef, Tar, and Weavile isn't anything new. Most Mega Latias builds have (or should have) answers for these mons.
Why would you ever let MLati take a toxic or a twave? I've heard countless times that it's useless when statused, but you wouldn't let your Zard-X take one either right? Or your Medi/Lop take a will-o? Many mons get shut down by status so I'm not sure why you would even let MLati take one of those or stay in on something that threatens it out. I usually pair it with something like Clef/Gliscor for status absorption. The whole argument about MLati taking status just seems kinda dumb to me and I don't think belongs.
MLati isn't supposed to check the entire meta either. It's like people are disappointed when it can't singlehandedly check Keldeo, Thundy, Lop, Medi, Lando, Drill, and so on. It's good at what it does and if you're slapping it on a team without proper support for it and expect it to be your only check for 20 different mons then you're using it wrong.
The moveset for the CM set isn't set in stone and I'm not sure why people insist on Dragon Pulse/Psyshock being the only two moves it can run. I never even run dragon pulse lol, and depending on what the team struggles with you have a variety of options - tbolt, ice beam, surf, stored power, etc, which still makes it unpredictable.
What's so bad about MLati being passive? I mean I get that this can be a drawback, but I don't think (if it drops) that this should be a reason. It's primarily used on BO/balance builds where a lot of other mons are passive. Being passive isn't a bad thing as BO has proven to be successful for some time now. It's not like a lot of mons can just freely switch in and take opportunity/momentum off the fact that it's passive (Clef aside but you should have an answer for that).
Reflect Type + BoltBeam is also a good set that lets it beat Ferro, Heatran, Skarm, and even things like Bisharp and Tar. Tar can't 2ohko after a reflect type and Bish usually gets 2ohkoed by tbolt after rocks regardless/can't switch in. I love this set and I'm not sure why people put it off to the side and regard it as useless. It also lets it check sand very well, being an answer for Tar, Drill, Keldeo, and most fat grasses that are usually paired with sand builds (unless you just let it get spored). It can check the waters it's supposed to check (Volc, Rotom, Keld) and stuff like Zard-Y + Tar with the reflect type set. Also not sure how Mega Heracross 6-0'ing most of these builds is relevant as I can't remember the last time that thing didn't massacre a whole team in the first place.
It's also very important to remember that this thing still boasts 110 speed, which lets it be a fat wall that can outrun Keldeo, Chomp, base 100's, etc...
I might update this with explanations of MLati teams that I've used and are still successful on the ladder, but you can check out these replays bc I've used the teams on there/slightly modified with great success still:
http://replay.pokemonshowdown.com/smogtours-ou-118622
http://replay.pokemonshowdown.com/smogtours-ou-138988
http://replay.pokemonshowdown.com/smogtours-ou-139608
http://replay.pokemonshowdown.com/ou-387992843
All those replays show solid MLati builds and how they handle different teams.
Can we also please get over how this thing "needs several boosts to provide offensive presence" or "you have 0 offensive presence if you don't use CM"? If you decide to use the reflect type set, then pair it with something like Keldeo or something else that provides offensive presence. I'm also gonna drop these calcs here to compare power, so don't take them out of context (I know MLati will never run draco):
+1 0 SpA Mega Latias Psyshock vs. 0 HP / 4 Def Keldeo: 374-444 (115.7 - 137.4%) -- guaranteed OHKO
252 SpA Life Orb Latios Psyshock vs. 0 HP / 4 Def Keldeo: 374-439 (115.7 - 135.9%) -- guaranteed OHKO
+1 0 SpA Mega Latias Draco Meteor vs. 0 HP / 4 Def Keldeo: 306-361 (94.7 - 111.7%) -- 68.8% chance to OHKO
252 SpA Life Orb Latios Draco Meteor vs. 0 HP / 0 SpD Keldeo: 302-356 (93.4 - 110.2%) -- 62.5% chance to OHKO
+1 0 SpA Mega Latias Draco Meteor vs. 252 HP / 224+ SpD Mew: 207-244 (51.2 - 60.3%) -- 90.6% chance to 2HKO after Leftovers recovery
252 SpA Life Orb Latios Draco Meteor vs. 252 HP / 224+ SpD Mew: 204-242 (50.4 - 59.9%) -- 83.2% chance to 2HKO after Leftovers recovery
+1 0 SpA Mega Latias Psyshock vs. 252 HP / 0 Def Heatran: 82-97 (21.2 - 25.1%) -- possible 5HKO after Leftovers recovery
252 SpA Life Orb Latios Psyshock vs. 252 HP / 0 Def Heatran: 82-96 (21.2 - 24.8%) -- possible 6HKO after Leftovers recovery
+1 0 SpA Mega Latias Psyshock vs. 248 HP / 252+ Def Rotom-W: 118-141 (38.9 - 46.5%) -- guaranteed 3HKO after Leftovers recovery
252 SpA Life Orb Latios Psyshock vs. 248 HP / 252+ Def Rotom-W: 118-140 (38.9 - 46.2%) -- guaranteed 3HKO after Leftovers recovery
+1 0 SpA Mega Latias Draco Meteor vs. 248 HP / 0 SpD Rotom-W: 265-313 (87.4 - 103.3%) -- 25% chance to OHKO
252 SpA Life Orb Latios Draco Meteor vs. 248 HP / 0 SpD Rotom-W: 261-308 (86.1 - 101.6%) -- 12.5% chance to OHKO
I fail to see how one Calm Mind is considered several boosts, as you're already hitting as hard as LO Latios after one, which isn't hard to get with how many things it forces out. I could drop these calcs for days but I hear - countless times - that MLati hits like a stick, which is completely false. Are you expecting it to hit as hard as Medi? It pulls its own weight, as the calcs show.
Lastly, not only are you already hitting decently hard (as hard as Latios) at +1, but I think people fail to realize how incredibly fat this thing is at +1:
252 SpA Mega Diancie Moonblast vs. +1 248 HP / 8 SpD Mega Latias: 168-200 (46.2 - 55%) -- 60.5% chance to 2HKO
252 SpA Life Orb Latios Draco Meteor vs. +1 248 HP / 8 SpD Mega Latias: 260-307 (71.6 - 84.5%) -- guaranteed 2HKO
+2 252 SpA Life Orb Thundurus Hidden Power Ice vs. +1 248 HP / 8 SpD Mega Latias: 156-185 (42.9 - 50.9%) -- 2.3% chance to 2HKO (and most NP Thundy are lefties lol)
Yes, these calcs are relevant (maybe not in a lot of scenarios) because they've won me games before. I CM'd on Hippo on the Diancie switch, outsped it, and didn't even get 2hkod. The second one was against an HP Fire Latios (I was running TBolt + Psyshock) and at +1 the second draco doesn't kill after a roost. Not only does this thing hit just as hard as Latios at +1, it also EATS, even from Diancie and Latios. Most Diancie are running more attack nowadays for Clef so that helps too.
I'm not saying this thing is a god, but I definitely think it's comfortable at home in A-. It's not suited for B+ and even if Suicune does rise to A-, I'm not sure how this would push for a MLati drop.
Keep Mega Latias A-.
Now that I think about it brb gonna make a Mega Latias + Suicune + Clef team and never lose a ladder game