“I did not lose my medical license for nothing!”
In theory, so long as the Necromancer has access to certain reagents, they can indeed turn the decay of their corporeal form down to almost nothing, but it is inevitable, hence the Lich’s use of a Phylactery, and why that Phylactery is almost always an object or creation of non-biological materials, meaning that the ‘prison’ holding their soul will not decay.
Within the phylactery, their soul can manipulate the material world around them, generating a new body out of corpses, or even conjuring one out of raw matter if the Lich in question is skilled enough.
There’s also the issue that Forsaken can ‘renew’ themselves through their Cannibalize racial, and also by ingesting the mushrooms of the Plaguelands, which possess a unique necrotic taint to them that can heal a Forsaken, but the broken method of their Undeath means that, unlike the Undead of Maldraxxus, they cannot heal or grow stronger. They can’t bulk up, or get more dextrous, they’re stuck at factory default settings, and require magical items to get any form of physical gain beyond that.
It would be interesting to see if Calia’s transformation was intended to try and ‘heal’ the Forsaken in some fashion, since unlike them, Calia has all of her soul, whereas most Forsaken were, on some level, affected by the soul-stealing powers of the Mournblade, Frostmourne, which was the primary conduit for the Lich King, and through him, Zovaal the Jailer, to spread Undeath and gain servants in the Material Plane.
This explain why so many Forsaken have scrambled memories, their better natures and positive emotions are muted, if not absent, and why no amount of restorative magic ever seems to be able to revive them as one of the living, as to be revived requires a whole, intact soul, or nearly so, and the Forsaken are missing enough of their soul that the magic to revive them simply fails to take the desired effect, instead simply resurrecting them as an Undead no matter how often the process is repeated.