The reason why I disagree with this nomination is because the purpose between both Latias' regular form and that of the mega serve generally different functions and can only be compared within type for the most part. Here are some basic differences between the two. Obviously I'm missing more but it's just a general idea why only so much can be compared. It should be established that one isn't necessarily better or easier to implement than the other.
Mega Latias' famed Stored Power set is stopped cold in its tracks by various threats which include but are not limited to M-Gallade, Bisharp, Tyranitar, strong physical attackers such as M-Salamence, M-Pinsir, M-Scizor, etc. The fact that regular Latias can't use this set as effectively is both a blessing and a burden, as it provides more leeway in what tools it can provide to its teammates for them to function and as such isn't as easily threatened by these various threats depending on coverage. The team itself is encompassed around M-Latias to handle these issues which as such focuses the teambuilding aspect of basically protecting M-Latias. This now puts more constraint on teambuilding in making sure your win con is able to achieve said win. Latias on the other hand is generally considered a component towards a win con either from a core perspective or the win con established by an individual mon. This can be done through the use of Defog to remove detrimental hazards or Healing Wish, which is even more of a blessing now to the offensive mons who appreciate this support due to a lack of self recovery from burn or being worn down. Due to the nature of why you would consider using M-Latias in the first place, it's safe to assume those mentioned support moves are off the table as the focus is making M-Latias win herself, not the teammates so to speak. The only thing really going for M-Latias over Latias besides bulk and the obvious Stored Power set to a degree is a 3 attack/Roost set which at that point you would be better off using M-Latios or even LO variants of both Lati twins due to a much more immediate offensive presence provided by them. It can also use non Stored Power and non Sub sets more effectively as well due to bulk but this has its own obvious issues such as being prone to status.
As far as the Knock Off thing is concerned that isn't necessarily true simply cause of M-Latias being relevant. For example your standard Ttar sets are still standard and M-Lati hasn't really changed this. The same things that threatened Latias are still threatening to M-Latias, M-Latias just has a little more bulk to survive a bit longer or sweep also hiding behind the fact it's running much more defensive spreads than regular Latias would. So in reality there is still indeed a cost to using M-Latias equal to more or less regular Latias based more on the different functions each one serves. Latias can be implemented while not taking up the mega slot and can provide ample utility to its teammates very efficiently in the meta so it's definitely fine to keep it at A+.
Well, then let me ask you following questions:
1. Does the cost of Mega Slot takes away the viable option of running a supportive Mega Latias set with Defog etc.
2. Is the Calm Mind set really limited to use Stored Power just because of the said win condition forces you to run "only" Dark Checks in your team to ensure your sweep?
3. Didn't I mention that if you care about damage output more than reliabily boosting your stats would make more sense to just use Latios in the first place?
To explain what I mean, I will address those points first.
1. I get that a Mega, at least most of them, are made to build your team around it and ensure a centain win condition instead of simply supporting fellow team members. Adding moves like Healing Wish or Defog take take the win condition of your "main" pokemon (your Mega Latias) away but in exchange you are allowing another member to stay healthy instead.
Now I have seen Healing Wish Latias being used as a win condition for mainly sweepers. You are willing to give away a team slot to create a win condition with your said Mega Salamence, Mega Charizard X or Mega Gyarados.
Well, those 'mons are more appealing than just ensuring that a regular Gyarados gets its win condition hence why you
might not want to use healing wish on Mega Latias (guess you can call that an oppotunity cost).
But nontheless, Mega Latias is still a great Defogger, it can still use a fast Wish and last time I checked it learns Dual Screens, Magic Coat or Tail Wind and is the fastest and bulkiest users of those moves, wouldn't you agree?
2. I get what you are saying and know that you are not trying to imply that mono attacking Stored Power sucks, but that really seems that you are saying that it is the only viable Calm Mind set and you didn't care to address Dual STAB or 1 STAB + Coverage Move or Lure Move of your choice which has completely different counters/checks for the most part than the Stored Power set.
And yes, this made the win conditions between the both Latias different but that wasn't always the case.
Calm Mind Latias was less viable in the XY Meta compared to previous gens, but it was still a thing until Mega Latias came (which is the point I am getting at).
That Calm Mind Latias did not rely on Life Orb back then, so why does that seem like that it actually does according to your second paragraph at the end? Having to run more defensive spread does not matter as long as you can ensure your win condition compare to getting worn down with each of your moves, because you are locked into offensive pressure.
For instands lets take your Roost + 3 Attack Latios as an example, you can get off a Roost with your Latios to bring it up to enough health to abuse your Life Orb, but how offensive the Meta is right now, you have to rely more on prediction or just give up your Latios by going for a last Draco Meteor.
It is like you said, you use Mega Latias for its bulk over Latias, not for its damage output, but, at the same time, you can be using Latios for its damage output over its bulk unlike Latias. Do you get where I am getting here? Which would explain my 3rd Question.
And the last thing is, I said that most things will go Crunch > Knock Off and what I meant was, MOST things that use Knock Off over Crunch might go for Crunch instead (things that can actually learn both). I didn't know that Ttar can actually learn Knock Off but Ttar that is one exeption.
I was mainly talking about things like Thunderus or Crawdaunt mainly while Thundurus might just go Special without Knock Off for the most part.
But I guess I can't say much against Healing Wish so I can understand why people want it to keep A+, but I still believe both need to be at least on even ground.