Okay, consider the following. Thanks to the necromancer The value of labor drops from the masses, but the profit goes solely to the necromancer. 

Even if the necromancer willingly allow themselves to be taxed.There is no guarantee that the taxed money goes to the masses. 

Thus, manual work become unsustainable as a source of income for the average individual. Leading the Necromancer likely a monopoly/ near monopoly status. 

People "May" potential see a drop in prices in goods. But what likely is to happen actually is mass unemployment. As people are forced into higher skilled trades and jobs that cannot be as easily replicated by the labor provided by the necromancer. Some people "May" be able to compete if they went out of their way to study necromancy themselves. But for most people, they'd simply become obsolete from the new economic system.

Thus civil unrest. 

This can be mitigated, through a system where the Necromancer pay for the Job displacement that was cause due to their "Innovation" in the labour industry. But it is neither in the Interest of the Taxation System nor the Necromancer themselves to do so.

reasons may include. people receiving handouts will be encouraged to work less and try to work the system by making more babies. or something like that. 

So for the common people relying on labor, they are  F\*\*\*ked. Now why does this sound familiar to real life.

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What do people do Now?!?! 

Either all the obsolete's eventually disappear which means, all the necromancer did was reduce global population. Which actually in the very long run disrupt their own source of labor, since there wouldn't be bodies to expand.

Some sort of weird middle ground where there is segregated dystopian society with a major gap between the have and the have not whose economies work independently of each other, whilst economical class change is near impossible, due to borders or something.

Or Somesort of revolt eventually, crushes the necromancer and the political and military taxation system.

Why Necromancy wouldn't work