Stymie Jackson I'm not ignoring that they get 24". I'm saying that you place it in your deployment phase and your opponent has full control from that point on.
Deploying it at the edge of your deployment does not guarantee a wizard never leaves their deployment zone, it means they'll walk, ride, or fly around that radius and cast spells where you can do nothing about that.
Compare that to a Runesmith for 75 and 2 RoSpellbreaking. Deploy him in a block of infantry in the same spot as you're imagining that anvil deploying. Turn 1, the block moves up 6" and is "pinning" the wizard, just like the Anvil does. If they move around the radius, the Runesmith can respond to that by moving another 6" towards the wizard, getting into 18" range so any spells cast in the command phase can have Ro Spellbreaking done. If they want to scoot further away from the battle to cast conveyance, vortex, and magic missiles, you're at least forcing them to continue to make choices.
I completely agree with you on the volume of spells being way higher. Knowing that and considering our generally poor magic defense, we shouldn't think about anti-magic like we used to, where we just try to crush the efficiency of a magic phase. We don't have the tools for that (I would kill for the Infernal Puppet WoC have access to). Instead, we can only hope to stop the important spells at the decisive moments. Timing and position is everything and the Anvil is awful at both.
I think the Anvil works best if it is about 2/3rds one way or another along your battle line, with a Runesmith in the opposite position. The Runesmith and unit advance and try to push wizards either towards the Anvil or towards the corner.
I don't think the Anvil is a bad model, I just think it is an incomplete solution to the problem of anti-magic. I think Runesmith with 2x Ro Spellbreaking is our best tool. Cheap, effective, but not bloated. I suspect 3x RoSpellbreaking has diminishing returns in some games because he'll be in combat and unable to dispell until later in the game IF he's alive still and IF stopping the spell matters enough to pay to make it automatic. Runelords seem bad because they cost more than a lvl 2 wizard and can take the same number of RoSpellbreaking as the heroes.
The Anvil with MRoBalance (and maybe 1 RoSpellbreaking) combined with the obligatory Runesmith might be worth it.