Would you like to play as a Necromancer with the ability to upgrade Vampires?

2. the number of skeletons raised will decrease

completely disagree... even with 20% necromancy, 2 left-side heroes with upgraded vampires will raise more skeletons than now...
i.e. what we get:
1)now: Gosha with 170(c) skeletons at 111 and throughout the entire 1st week crushes everyone and everything, raising N skeletons.. well let's take into account chains etc... therefore-other necromancer heroes raise 1/2*N skeletons.. (but mostly even more than Gosha, so let it be 2*N)...
2)after the vampire upgrade: Gosha with 170(c) skeletons crushes everyone and everything... + in addition to this, during the 1st week vampires are upgraded and another necromancer hero clears the remaining walkers.. well also with tactics and shooters.... and gets as standard another 2*N or even 3*N and 4*N
what's the result.. by the 2nd week.. there ГО will already be lying down with 1000+ skeletons..

3. the importance of Galthran will be weakened

galthran, as he was the daddy, so he will remain.. apparently, Cloak of the Undead King is rarely assembled - so they raise skeletons.. and you can't get away from him..