Originally Posted by NeoSeraphi
I don't think Yitzi is worried about the extra 6th, 7th, and 8th level spells per day. I think he's more concerned about the +3 bonus to all save DCs (+4 if you include old-venerable age, +5 if you're a race that has a +2 bonus to Intelligence)
With DCs, every single point counts, because it's a potential 5% increase in every one of your offensive spells succeeding. That +5 there...that's a 25% increased success rate. That's incredibly significant, much more so than a +7 bonus to damage with a two-handed weapon (Though that itself is also pretty significant considering it applies to every attack roll)
Plus, you have to consider epic levels. Epic spellcasting focuses on Spellcraft checks. Every point of Intelligence increases the threshold for more and more powerful spells you can create and cast. (That +5 bonus? That's 5 more points I could spend creating a spell and still take 10 on my Spellcraft checks to cast it)
Let's look at some monsters, since players mostly fight monsters in a standard game.
A Wyrm Black Dragon is CR 20, so the party should be able to take 4 of them in a day and be pretty beat up/exhausted of resources. That means a OHKO shouldn't be feasible, but it should be possible. Saving bonuses are +26/+19/+23. Core doesn't particularly have Ref or Die spells, so we will consider a hypothetical Fort Save or die spell, as well as a Will or Die spell. DC for a 9th level spell from our 20th level wizard is 10+9(level)+9 or 12 for a total DC of 28 or 31. Dragon makes that on a 5 versus an 8 assuming a will save, and that is ignoring its SR of 26... which should mostly be ignored as it will be beaten by the wizard on a roll of 6 on d20. There's about a 14.5% chance of the dragon dying to the wizard with an ability cap versus 29% chance of dying to the wizard without the ability cap, assuming the wizard knows to use a will save spell and that there is a will save or die spell at 9th level (there's Imprisonment, which is a touch spell, and Dominate Monster, which is pretty nice to land on a dragon).
A Balor is also CR 20. Saves are 22/19/19 with 28 SR except it has Unholy Aura at will so saves are really 26/23/23. That's better than the dragon wiht better SR, so the wizard has even less of a chance of ending the encounter in one spell.
The Tarrasque is CR 20 and has saves of 38/29/20 with SR 32 and immunity to rays, lines, cones, and magic missiles. Again Dominate Monster would be cute, but again the odds are not great: even the uncapped wizard has DC of 31 so Terry needs to roll less than an 11 and the wizard needs to roll a 12 or more. Possible? Yes, but it is worse than a coin flip and Terry can kill at least one person a round given the chance.
Yitzi isn't planning on including racial bonuses in the cap, and we haven't heard if old age applies or not, so it's at most a +4 difference.
I didn't consider Epic because, honestly, Epic is even worse at being balanced than pre-epic. I also didn't consider Epic Spellcasting because it is terrible without mitigation and with mitigation Spellcraft is irrelevant. I posted in this old thread
to explain why.