Fair enough, I will concede to you the semantic argument that the Master Unit List labels them as Juggernauts, that's the word of God, I won't contest you there. But come down to my level and speak to me using my Lexicon for a moment. My definition of Juggernaut is defined as the role that the stock Atlas plays, it being in a tier to itself among all of the stock mechs, as you've conceded.
This decoupling of front and rear armor pools allows lighter custom Assaults to enter that role and join the stock Atlas: A max armor Stalker for example, sporting 18 SRM tubes, 6 medium lasers, and an LRM15.
At the risk of comparing e-peens, I believe I have tested this specific issue quite (perhaps more) extensively. I've got 700+ hours between the beta and futzing with custom JSON files. Nothing stock besides the Atlas quite achieves the effect of "wow... I'm shooting at this thing, and like nothing is happening... what am I supposed to do?".
Which, to be fair, is a role that's supposed to be fairly unique to the Atlas. Either way, the overall difference in max possible armor between an 85 tonner and a 100 tonner is a single Large Laser's worth of damage. And even accepting that nothing can really compare to the Atlas, that doesn't mean that this is a problem. Or that removing one of the major components of mechlab strategy is the best solution. Lighter mechs shouldn't be as tanky as heavier mechs, otherwise what's the point of them being heavier?
I'm not at my main computer so I can't pull up precise numbers for how much time I dumped into the beta, but I no-lifed it fairly hard. Not to the same level as you, but enough to say I know what I'm talking about.
Well A) you can't actually raise that rear armor past 24.
B) I don't think you lose all that much strategic value. Because frankly there is already plenty of opportunity cost elsewhere. This change doesn't lead to broken OP mechs, it lets Stalkers fill the same role as stock Atlases without giving them an insane amount of rear vulnerability that would make them non-viable.
Or you can just drop a missile launcher or two, or change the weapons loadout to something more focused, to free up tonnage to do the same. Using TT numbers, at a reasonable rear armor of 10 points the difference between the CT of the Atlas and the CT of the Stalker is 52 versus 44. That's not an excessive amount, especially not to the point where it would exclude the Stalker from taking a Juggernaut role.
Also, I thought the OP dug up something saying that the rear armor can mount the exact same total possible amount of armor as the front at the same time? Is this not correct? Because that's how it reads, and how everyone has been treating it.