Mages are also gated by CDs, Assassins can always fall back on there AAs, which also generally hit like trucks late game. You can make the argument that Mages are better team fighters while assassins are better duelers. In a season where damage is outrageous a lot of fights are over in 4 seconds, which is the general CD on most Mage abilities outside of a few select abilities.
Also granting AP champions a item like a reverse maw comes with a downside, what item do you sub out for this item? Most Mages core build these 4 items -
with these items being the situational that get rotated in and out -
. Most champions default to
. So it seems that a reverse Maw would usually sub out with Zhonyas or Morello, but losing Morello means a loss of needed damage and penetration while subing the passives of a reverse Maw and Zhonyas is kind of a weird trade.
Assassins and Bruisers also build tank boots more times then not, so that's another F U to mages, and most AD assassins can usually out lane their mage counterparts (Talon, Zed, Yas (I understand he's not a assassin, but close enough for this)) Mages can also build tank boots but they lose out on a chunk of damage for it, and again are more gated by CDs and mana (early) then assassins.
So if I build Tabis, and they get a Hexdrinker, I can no longer burst them out in the window before they burst me out. If I have to grab a Seekers as well I can no longer do anything but farm, and even then they can prolly just dive me due to turrets being non existent after 5-7 mins.
Now I understand most lanes don't play out like this, if I'm not against Zed and feeling comfortable against Talon I'm not going Seekers or Tabis (both become irrelevant mid-late as they can burst through them just fine) while with a Maw I can't burst them out unless I'm stupid ahead, which would prolly be their 3rd item completion.
Zhonyas is useful, but doesn't do shit if my team isn't standing around me to back the assassin off while I'm golden, where as a reverse Maw would allow me to simply survive the all in for a few seconds to try and retaliate.
So, I'm not disagreeing with you per say, it would bandaid mages and make it harder for the Zeds, Talons, Yass, Irelias, Wukongs, Hecrims, etc to just dump out damage on me, it comes with a trade off.
AP and AD should be even, not a one sided bias towards AD.
Something like 60 AP, 40 Arm, the Shield would be fine, nothing overtuned, and don't give it CDR so it becomes a actual choice, as most Mages try to hit the 40% limit so they can cast more spells between being gated by CD.