The way I see it, I think people who get caught in this unhealthy swing of opposing extremes are carrying around a contradiction between what they are consciously trying to be and what they "need" to be on an unconscious level. These excesses can be a form of compensation. If someone does not have a very pleasurable life, they will seek excessive pleasures.

To me, you have put that well and to the point. It is what I meant. However, I see it as a failed attempt at balance because it is in a negative field of tension. So only two ends are addressed. If one expands the field, perhaps the following would result.

"Pleasure" or "feeling pleasure" is first of all a human peculiarity. If I exaggerate this pleasure, I find myself back on the underlying point of frivolity or "decadence".

But what could be the sister quality of joy/pleasure?

One could cite "seriousness" or "profundity" here. So that there is a positive field here between these two sensations: The lightness on the one hand, the profundity on the other. Now, if you overdo it with seriousness, if you overstretch the arc of such seriousness, you end up at the point of humourlessness, a joylessness. This is the negative sister quality of decadence/frivolity.

Who has not experienced a phase in which one was caught between these two negative qualities? It is difficult and extremely exhausting to live between humourlessness and frivolity. Decadence has a certain tiring boredom, in which there is no longer any real joy in the good and the beautiful and the true, because there is too much of it. Pleasure falls by the wayside.

That is why I treat it as a faulty balance, because it moves in an unpleasant field of tension from which one can only get out if one learns to find the way back between humor and profundity.