I never really doubted that stall would be good, as I'd imagine that's basically the main reason why ubers was chosen over OU in the first place, since OU Megamons would be more offensively inclined. But what's the difference between Megamons stall and ubers stall? Because quite frankly it seems like building stall would be even more restrictive with the insane power level.
I disagree, I feel like OU Megamons would be an even more hostile environment for stall since teams of full megas would not be uncommon (in the sense that they are viable; full megas teams are common now but they suck) , and the main cruxs of ubers stall that make it viable in the first place are obviously not present. I don't necessarily think this is inherently a good thing, of course, but stall being viable or unviable isn't really a factor of complaint when in comes to OMs.
But they are still just ubers. Making a mon slightly better in its own tier is hardly a shake-up, and quite frankly the ones you mentioned are still often outclassed except maybe Lucario who I admittedly forgot existed.
As for Ampharos and Camerupt over trick room, I honestly think that's silly. Even with its much lower speed, I'd much rather use a minspeed Pdon over Camerupt, as 99% of anything slower than you still loses to you anyway, and the vast majority of mons in Ubers who hover around the 70 speed tier invest enough to outpace uninvested base 90s anyway. I can't even imagine how Ampharos would do much of anything even under trick room, especially with the aforementioned Pdon sapping your momentum away from an archetype that dies with its momentum. To be blunt, the very idea is seriously a stretch, and I highly doubt it's validity.
Also, I can name way more megas than that who have virtually no reason to be used, and I think you could have too.
There is no good reason to use Mega Metagross over ultra Necrozma, no reason at all to use Alakazam over Deoxys-attack, and barely a reason to use medicham period with its mediocre speed and psychic terrain stifling priority. Yes, I suppose I could have used mmx, but how different is this from a regular ubers team, again?
Just because you technically can use all of them, doesn't mean it's a good idea, and indeed it's not a good idea in the majority of cases. More megas would absolutely be viable in an OU format and I think this is very clear and plain to see.
Honestly, very few megas are totally not viable. Like most of them are actually usuable in one way or another and making it OU won't change the viability of the bad ones. You will just make it so that the ones with a usage rate of 10%-30% now will have a usage rate of 50%-70% in OU. The meta will quickly centralize around the best megas and some will just rarely be used. It is inherent to this metagame because nu or ru pokemon can't become gods like in other oms, only megas can become better. So the best megas are used and the viable pokemon of the tier it's played in remain used.
I also oppose against making it OU because right now we have indeed a nice balance between offense and defense. You say OU would make it more hostile for stall but Megamons stall is legit OU or UU pokemon like Slowbro-Mega, Aggron-Mega, Chansey, Mega-Sableye, Mega-Latias, Mega-Scizor and Mega-TTar with very few Ubers (Ho-Oh and Arceus mainly). If megamons is OU, you lose Gothitelle/Mega-Gengar and you lose breakers such as Mega-Salamence, Mega-Lucario and MMX. Add to that the usual stall pokemon of OU. Have fun breaking stall.
In my opinion, if we make megamons OU, it will just be stall and weather wars and we will have to ban shit to make it balanced again and we will lose the opportunity to use other megas directly once again thus limiting the possibilities.
In the end, it's not up to me anyway but i feel Megamons is best played in Ubers because we can use all the megas and the bad ones won't be more used if we make it OU. Pokemon that aren't used much are not just bad stat wise but also because of their type and movepool which won't change even if Megamons is played in OU.
Concerning Mega-Ampharos and Mega-Camerupt, Trick room actually makes them viable. Like you are talking about some megas not being viable and you just straight say it's silly and doubt their viability. Yeah sure TR doesn't make them A+ rank but they can actually be good. Primal-Groudon is indeed annoying for Mega-Ampharos but Z move for example helps 252+ SpA Mold Breaker Ampharos-Mega Devastating Drake (160 BP) vs. 252 HP / 0 SpD Groudon-Primal: 375-442 (92.8 - 109.4%) -- 56.3% chance to OHKO. Electric/Dragon is actually a nice combination.
Mega-Camerupt does actually more damage than Primal-Groudon with Sheer Force and Life Orb. Let's face it, outside of TR it is bad but TR gives it a nice niche and the combination of Earth Power/Fire Blast is pretty good. It can also run Explosion for Chansey and special walls.
No reason to use Mega-Metagross? Ok.... Choice Band/Scarf Tough Claw with natural 350 Speed and 389 Attack. Access to priority or Rock Polish with Life Orb. Ultra-Necrozma is also very weak to all the pokemon that naturally outspeed it unlike Mega-Metagross be it Mega-Alakazam, Mega-Beedril or Mega-Sceptile.
I think you forgot Mega-Alakazam gets Trace tbh. This makes it counter Diancie-Mega to a certain extent, it can copy Adaptability on 2 megas that are used a lot or just other useful abilities like Regenerator on Ho-Oh or Parental Bond on Mega-Kangashkan. More bulk can also help against Special Mega-Lucario and Mega-Scizor that needs Choice Band for a guaranteed OHKO with Bullet Punch.
Finally, Mega-Medicham has 200 base Attack thanks to Huge Power. Give a Choice Band for insane damage or a Scarf for speed control. Like how can you say of a pokemon with the highest attack stat that there is barely any reason to use it?
From my experience on the ladder, only few megas are totally useless. Most have viable sets that give them some kind of utility.