You countered your own point. WHM's situational 10 seconds of Temp/Plenary/Cure III glory is worthless if nothing in the game requires it, while a consistent supply of short-cd oGCD's that can be weaved at low or no cost and cover every mechanic in a Savage fight are far more valuable. While leveling, WHM is even worse. At lv50, AST is basically a WHM with cards, more mana, cheaper heals, full mobility and ED has a much shorter cd than Bene.
Again you shredded your own argument by yourself. "WHM isn't immobile because it can burn it's own healing lilies, spam its DoT and ignore useful pieces of the toolkit like Tetra and Benison to assist weave". That's immobility. It cripples itself to do basic functions like move or use the full toolkit. Using Dia to move is an absolutely last resort because on top of the 180 potency loss, you misalign Dia with every second Assize. Benison and Tetra can be compared to Celestial Intersection and ED on AST, which we use almost on cooldown. With WHM it isn't because they don't have value, (18 Benison over a fight is a whopping 4500 potency of saved tank healing) it's because it isn't worth crippling yourself for that when your co-heal can handle it instead.
10% mitigation is huge. Probably at least as much as a Succor Shield on average.
Sacred Soil is so strong because it has a 30 cd. That means you can have it on demand for almost any raidwide you like, where Asylum doesn't have that luxury. Again, WHM has big abilities, like Temperance, Bene and Asylum. But things like Essential Dignity, Soil, Excog, Diurnal CU and so on heal or mitigate enough to handle even Savage mechanics and are available often enough to cover them all.
If you're not being outdone by your coheal, (particularly AST) you or your co-heal are playing poorly. An AST can heal significantly more than WHM, for free, so trying to reach an equal hps is basically wasting their heals. Even if a GCD heal is needed, you dump it on the AST because a lost Glare is 300 potency and a lost Malefic is 250 potency. And yes, even with all that, at high percentiles WHM has the lowest rdps of all the healers in this Savage tier.
As for your signature, a single good kill on the easiest boss to heal this tier in a speedrun group that skipped Pitch Bog II by a mile does not make you an expert WHM.