It should be put out there we still don't know *what* happened to Kel'thuzad, he kinda just buggered off after Naxxramas.
That said, you're sort of downplaying how bad Kel'thuzad rising to power was and playing up the minimal risk of people using Necromancy. The fall of Lordaeron can be solely attributed to the Kirin Tor's banning of Necromancy and extreme aversion to it. This led to Kel'thuzad going to Northrend, being taught Necromancy by the Lich King, and returning to Lordaeron. After this the Cult of the Damned was formed (backed by Kel'thuzad, who was described as having a fortune) and the Plague of Undeath was spread, with the Kirin Tor being unable to do anything -- even countering the plague was outside of their ability. There's nothing to show that this scenario would have played out if the Kirin Tor hadn't banished Kel'thuzad and shunned all knowledge of Necromancy.
That said, there's already a tacit acceptance of Necromancy in the Alliance and Horde because of Death Knights, and there's not been a massive increase in usage of Necromancy since Wrath. We're in the situation where we're hypothetically risking dozens of Kel'thuzads, and the only active Necromancers are the remnants of the Cult of the Damned that Kel'thuzad founded. Other than that, there is Sylvanas, but plot armor stops her from facing consequences.
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Sure, but the idea that Death Knights could lose abilities to Necromancers when their only overlap is thematic and not in terms of actual spells (there was no Necromancer hero class in WC3 to go off of) is the same concern that Engineering could be removed or lose patterns because Tinkers share the same, loose theme (but nothing else). The point is that both concerns are completely absurd.