I like Necroblaster being able to self sustain, and at is one of my favorite shadow cards.
I might be a noob, but the build, set up, and then forget nature of Necroblaster is core to how I use the tower in pvE.
With the requirement of corpse setup / using enemy corpses, it already feels shadow themed. Not every shadow mechanic needs to come with a major cost. A lot of the core ones (furnace of flesh, chaos altar, time vortex) instead involve cheating cost, which is a very shadowy thing to do. (e.g. making a pact with the devil, and then weaseling out of it), and I don't think making them unable to self sustain adds anything. It feels it just makes the card less fun.
Maybe if one could somehow set up a corpse network, I would like it not being able to self sustain. You could enable soul-splicers to link together, and then have one massive sacrificial area, that fuels all corpse using stuff.
For any given map, you can also easily tweak the enemy units being sent against a Necroblaster to make self sustain impossible. E.g. units with a high innate damage reduction, or high regeneration rate, or units that also use corpses. Necroblasters are going to have huge difficulty self sustaining against an onslaught of overlords for example.