Was Magic Resist overnerfed?

You do realize AP does equate to massive tower damage.

I understand that a ratio goes to tower damage, but AD champs get the benefit of building attack speed and/or armor pen into their build and having it be viable (similar to AP champs and mana regeneration), so it's still not even close. If you think Orianna, Lux, etc. can do anywhere near the damage of Zed or Jayce split pushing on towers then you haven't played very much.

However, AP already account for that factor by having higher stat values along with superior actives compared to the AD counterparts. Additionally, AP split pushing CAN be done now if the champion has a Sheen item, banner of command, or Baron Nasher's buff.

It has always been a given that AP scales much better than AD to give incentives for champions to build AP for damage if they have AD ratios as well. Nerfing MR while leaving armor mostly intact will only further widen the gap between AP and AD for better or worst.

By building sheen they are sacrificing AP damage for that specific effect, and its still inferior to an AD caster who can build armor pen and/or attack speed into their natural build, such as Zed, Jayce, or Pantheon.

AP scales much better then AD?

If you are talking pure numerical values then yea, I guess you win that case.

However, only a portion of that goes to any kind of auto attack damage, and only on certain targets (not champions) and there are very few viable items to mix attack speed into an AP's kit, still giving AD scaling champions a massive advantage in this area.

Zed is a bad example since he is overpowered, even though Zed players do their best to deny it and downvote anbody who mentions it.

Fine.

Jayce, Pantheon, Gnar, Lee Sin, Renekton, Regnar, Talon, Yasuo.

All these champions get the inherit advantage of their abilities scaling with their auto attack damage, and have just as much utility/clearing/assasinate power as AP champion counter parts.