Necropolis discussions.

It's not a given that the bonus weakens significantly; you can cast "Berserk" on their army, and raise your own undead with "Raise Dead," so playing against Necromancers is always challenging.

Hmm... Do you even understand what you're saying? The conversation was about the Necromancy bonus, not about spells. So, here's a well-known fact:
The longer the game lasts, the smaller the final bonus from Necromancy.
If you need to, I can provide numbers, if you don't want to calculate them yourself.