I believe the answer lies within the "self-casted polymorph" scenario.
While this may be slightly controversial, considering the JCraw response on the matter, I believe that status effects, such as Rage, Giant Might, and most importantly, concentration, can be, and are maintained while within a polymorphed form. This is strictly my RAW interpretation, and while I can definitely see how effects such as these would be deemed to be dropped RAI, the specific wording of these class features are the factor that answers this.
TLDR: I agree with your latter rationale. Rage is a status effect that, once granted, is only ended under circumstances specified within the effect itself. This is argued to be true due to the nature of concentration while polymorphed. While acknowledged and argued for, I would still follow DM discretion, as I think we can all agree that Rage, Giant Might, Bladesong, etc., would be OP on a T-Rex or Giant Ape.
For clarity on the argument, I'm labeling certain things as "status effects", these are, in essence, Conditions that are not labeled as such. There are benefits, such as Rage, that are active effects applied to a creature that are not specified in RAW as conditions. This is listed here to provide contextual links between these benefits mechanics of the game.
For the rest of us who've read way too much into this, let's look at the specifics of Polymorph:
The target's game statistics, including mental ability scores, are replaced by the statistics of the chosen beast. It retains its alignment and personality.
So if losing the classes' features implies that all benefits gained from them are removed upon being polymorphed, why is it possible for someone to self-cast the spell? This is because there is a spell effect cast upon them, at which point the caster gains the status effect of "concentration", and the only factor stopping the status effect of "concentration" is specified as such:
If a spell must be maintained with concentration, that fact appears in
its Duration entry, and the spell specifies how long you can
concentrate on it. You can end concentration at any time (no action
required).
Normal activity, such as moving and attacking, doesn’t interfere with
concentration. The following factors can break concentration:
Casting another spell that requires concentration. You lose
concentration on a spell if you cast another spell that requires
concentration. You can’t concentrate on two spells at once. Taking
damage. Whenever you take damage while you are concentrating on a
spell, you must make a Constitution saving throw to maintain your
concentration. The DC equals 10 or half the damage you take, whichever
number is higher. If you take damage from multiple sources, such as an
arrow and a dragon’s breath, you make a separate saving throw for each
source of damage. Being incapacitated or killed. You lose
concentration on a spell if you are incapacitated or if you die. (From
PHB Concentration)
This is the main hole in the argument for status effects being removed if class features are removed, because concentration is tied to spellcasting (since it's a part of the duration of a spell), which is removed when polymorphed. If the form you take doesn't have spellcasting, how does it know how to concentrate on something it doesn't inherently understand? Not to mention the specific section of Polymorph:
The creature is limited in the actions it can perform by the nature of its new form, and it can't speak, cast spells, or take any other action that requires hands or speech.
If a creature is polymorphed, they lose access to the activation of features, just as they lose access to the activation of further spells, but they do not lose access to the status effects provided by these features during (and prior to) the casting of Polymorph, unless a character causes them to end early as specified within the effects themselves.
The way I see it is as follows:
- Spell/Feature is activated as specified within the spell/feature's description.
- Once activation is completed, a status effect is established (in a spell's case, both the spell effect and the status effect of concentration are placed upon the creature/s).
- At this point, access to the spell/feature is no longer relevant to maintaining this effect, as the conditions for maintaining are established.
Example of this in action, starting with, "Activation":
In battle, you fight with primal ferocity. On your turn, you can enter a rage as a bonus action.
Simple, you use a bonus action during your turn on a round, the feature is used to bestow the status effect of, "Rage".
Now the effects and duration are in the phase of, "Establishment":
While raging, you gain the following benefits if you aren’t wearing
heavy armor:
You have advantage on Strength checks and Strength saving throws.
When you make a melee weapon attack using Strength, you gain a bonus to the
damage roll that increases as you gain levels as a barbarian, as shown
in the Rage Damage column of the Barbarian table.
You have resistance to bludgeoning, piercing, and slashing damage. If you are able to cast spells, you can’t cast them or concentrate on them while raging.
Your rage lasts for 1 minute.
The effects bestowed have been established for the status effect, "Rage".
And finally, you begin following what you must to "Maintain" the effect:
It ends early if you are knocked unconscious or if your turn ends and you haven’t
attacked a hostile creature since your last turn or taken damage since then. You
can also end your rage on your turn as a bonus action.
You can see this same ability structure throughout several other features. While all passive effects are removed, as the feature is no longer on your sheet, ongoing status effects are never specified within the description of Polymorph itself. And while Wild Shape does specify that keep feature benefits, Polymorph never specifies that it removes active benefits, simply that access to the features are removed. So if you lose Rage due to not being attacked, attacking, or becoming incapacitated, you lose the status effect and cannot reactivate it until you regain access to the class feature, just in the same way that concentration is kept when polymorphing one's self, but you cannot cast any new spells until you are out of the new form.
But I will end this by saying that at the end of the day, it's entirely up to your DM, and it's certainly unfair to expect a DM to take this as the laid down law of the land. I mean, 136 HP with resistance to bludgeoning, piercing, and slashing damage (in the case of a Raged T-Rex) is pretty busted.
Secondly, for those arguing the use of items to cast spells prove that concentration is not specifically tied to spellcasting, you're proving my point on the matter of the effect of Rage not being tied to the access of the Rage class feature. When you use an item to cast a spell, you gain access to the Activation of the spell specified within the item. After the effects of it are Established, the only thing you need to follow at that point is what you need to do to Maintain the spell, regardless of your character's access to the Spellcasting feature. The same can be said about the effects of Rage, or Bladesong, since it follows the same structure described within the mechanic of concentration.