I don't think that summoned units on the battlefield should degrade over time.
My guess is that they did this change because of multiplayer and I can understand that.
But for the campaign there is no good reason for it in my opinion.
Vampire Counts, Vampire Coast, Tomb King, Skaven & Lizardmen have summoning spells or abilities, giving them a tech research so summoned units don't degrade over time or for the summoning spells last skill upgrade they stop degrading, That way it won't effect multiplayer.
Summoning spells feel very underwhelming right now I rather use my winds of magic for wind or vortex spells. This small change would make summoning spells useful, right now I feel that they are not worth casting.
Some lords have strong summoning spells like Helman that can summon Grave Guards but i think it is fine that you can do that in the campaign.
"It is said that Abhorash wept tears of blood for his victims
and from that day on he travelled to the desert to pray upon the scattered nomads
instead of his own people."