One of the side benefits that the new summoning system brings; Tzeentch summoning is based on successfully casting spells, both theirs and their enemies'. By moving our summoning to be instead of spells instead of done by actually casting spells, we just became a serious middle finger to Tzeentch players who were probably expecting to get a lot of their own summoning out of facing us. We now have the ability to shut down all their spells while foregoing our own to summon endless hordes of units, using the same Slann/Astrolith/Skink heroes package that we've always had.
I'll admit, anti-Tzeentch is a pretty niche thing to add to our army, but I doubt many others will do it better.
As for the idea that we "won't be using the new realm or endless spells due to needing the slots for summoning," piffle! Every wizard in the army learns those spells, not just the Slann. Astroliths buff every wizard in the army, not just the Slann. Not even just wizards within a set range! Any Starseers/Starpriests you have out in the field to extend your Slann's reach could be casting those spells themselves, and I can't see Starpriests becoming really expensive. Admittedly, all of that's subject to change (especially the Astrolith, I can see it getting nerfed somewhere and it'll probably be the wizard buff being global) but assuming we don't have the whole casting element of our army rewritten, it ought to mostly hold up.