Of course it would make sense to add them now, but that wasn't your original question. You asked what expansion they would work in other than a Death expansion.
The answer is they would make sense in any expansion after Shadowlands, given that this is a place that establishes a greater, ancient source of Necromancy that we have never seen before. It's been confirmed that this place will be a big part of the story moving forward, into other expansions as well. This isn't gonna be a typical one-and-done deal, and Blizzard established this from the beginning.
Cuz you have Covenant powers to fill in that role. In a sense, we don't need a singular class to guide us into the Shadowlands because we're getting 4 prominent lore characters returning, and we get to choose which one will be our guide.Why wouldn't the factions call on Necromancers NOW to allow them to bring them back and forth from the Shadowlands? Wouldn't it also make sense to have Necromancers (literal scholars of death) be our guides in the Shadowlands? Your reasoning doesn't make sense.
And it is like asking why wouldn't the Alliance and Horde call on Tinkers to fight in any of the previous expansions. Well they totally could have, but they didn't, and it doesn't mean they can't in the future.