They lowered AP totals across the board so they could increase the passive on Deathcap in order to make Deathcap a desirable item. I think the numbers are fine right now and that this wouldn't be worth doing.
Well they failed, because Deathcap is currently a terrible pickup in the majority of games.
Yes AP numbers are fine, while Morello's, Void and Rylai's are broken. If you nerf these 3 items, which needs to be done AP numbers would no longer be fine.
I'm pretty sure that Luden's gives you more raw damage on any champion as a first or second buy. This makes it a strong pick up on assassins and other single target burst champions. Deathcap outscales it later when you have more items for the passive to work with. It is also a fair bit cheaper, making it a much better rush item.
No single item gives you more damage than a Voidstaff. Against a 57 MR target, which is flat runes and a Locket buff you increase your damage by 9%. That's all of your magic damage though, bases and ratios, in basically the lowest realistic case for MR. Plus you get the 70 AP on top of that and it costs 550 less gold, so you can throw an Amp Tome on top of that for 90 AP. What that means is for Luden's to ever be better its passive and 10 AP need to do more than a straight 9% increase to damage. Do the maths on any champion, it doesn't. Two ability hits will normally make Void better. To add to this there's always going to be champs with more than 57 MR, so Void just wins out huge.
Deathcap is just too expensive. Void costs over 1k less gold than it and is often actually a bigger swing in damage. Once again the minimum that a Void can increase your damage by is a flat 9%, Deathcap is a 35% AP boost, the actual flat AP numbers are about the same since you can throw a NLR ontop of your Void for 100 gold for a total of 130 AP, 10 higher than Deathcap's raw amount. The difference here is that a 35% AP boost is actually bigger than the 9% damage boost, just. But the second someone picks up something like a QSS or isn't a glass cannon Void+NLR is far better than Deathcap.
Oh, by the way, did I mention that that 9% assumes no Sorc Shoes? If you have them it's actually a 12% difference, at which point you can start to question if it's better than the 35% AP from Deathcap. If you're running flat pen Reds and the Mpen mastery too it's 14%, at which point it is better than a 35% increase to AP without contest, and this is still in a best case scenario for Deathcap vs a low MR target.
It's only too good in the absence of anything to compete with it. Athene's was taking away as a competing item, and Tear builds will never be good on everyone making Morello the only choice for mana sustain on most champions. Imo they need to either make something that has as good mana sustain, but with some other utility asides from Grevious Wounds, or they need to remove the Grevious Wounds from Morello and make a new (niche) item for AP champions with it.
No, it's straight up too good. It's a 112% gold efficient item which has the highest AP of any current non Deathcap/Soulstealer item. There's pretty much no reason not to build a Morello's right now, even if you're going Tear it's still a solid item because it's 100 AP and 20% CDR, that's never not good. If you removed the mana from Morello's it would still be a fairly good item, that's how out of line it is right now.