Sound reasoning--I agree. Since coming to the Boards, I've noticed that much of the community has a penchant for anything underplayed and unbridled hate for assassins and whatever seems strong.
I try not to debate with people about balance since I couldn't know whether a champion is strong based on playing against that champion (I'd have to play him or her in many matchups and teamcomps), and because my champion pool is pretty small and is composed of champions most people don't complain about, I can't give any valuable input.
Now people are finally complaining about champion that I play, so I can finally talk about balance. However, it seems like most people don't actually understand Karthus's current state of balance, and instead jump to his aid and ask for buffs because he's underplayed and he's very docile in the laning phase.
(Karthus seems to be a damsel in distress to the Boards, needing to be saved by everyone.)
Needing a reason for the proposed buffs, they call out the one thing that they actually know is bad about Karthus: "he's countered so hard by mobility".
Karthus's designed weakness.
Karthus was countered by mobility before the wall nerf. In fact, his lane matchups haven't changed since the wall nerf. He still does trash against mobile champions with heavy kill potential, and he still can win matchups versus immobile opponents.
The one most important thing that the nerf did was lower his early game kill potential. Late game, the decaying slow matters little since a slow that strong even for a small amount of time is enough to catch someone out.
Karthus got a ton of damage with preseason in the form of Deathfire Touch. It was amazing--he had a spammable single target spell and a damage over time spell. He could abuse the crap out of the mastery, and deal waaaay too much damage. He'd be able to set up jungler ganks very easily with his strong wall, and the opposing laner would likely be quite low since one critical Lay Waste did a lot of damage. He could also dissuade all-ins on himself, because he'd be able to match the damage that the opposing laner put in him, and if the opposing laner went all in, he or she would be low enough for Karthus to finish him or her off with Death Defied.
This meant that Karthus would normally be going either neutral in lane because of his safe laning or positive because he was able to set up a good jungler gank. In either case, he'd be up in cs because of his farming mechanics.
This was not a healthy early game for a hypercarry.
So they nerfed his wall. He still dealt nice damage, but he couldn't apply it as easily or set up jungler ganks as well.
Honestly, he's probably better off than he was before preseason, since even though his wall was nerfed, he's got the amazing Rod of Ages and Deathfire Touch to augment his laning.