...except cdr is a stat roughly every champion (except for non-caster adcs both melee and ranged) cares about. Tanks, supports and AD assassins (such as nocturne or rengar) care greatly about CDR and will itemize heavily into those. It's not a "ap mage stat". It's an everyone stat.
and there's a thing called armour pen you conveniently left out of your post, despite bringing up mag pen as a stat that matters. And comparatively, there is substantially far greater armour items than there are MR items to deal with. At least try to make your post sound unbiased?
and then there's the lifesteal- What exactly do you mean by return that damage so I can do more damage? lifesteal doesn't uhhh.... do return damage? It's not thornmail. It's lifesteal: your attacks provide health based on damage dealt. It's a defensive tool similar to buying health regen on a champion (except it requires a victim to attack). It's a defensive sustain option just like how max mana/ max regen provides sustain for a caster (I hesitate to say mage, since again, tanks and supports do care about mana as well and also jayce who I would class as not a tank or a support). And the comparison between lifesteal and mana regen becomes even greater on certain casters, where mana/regen becomes literal sustain (healing) when certain abilities are used (such as swain or ahri or soraka).
Also, here's a link to the wiki
http://leagueoflegends.wikia.com/wiki/Mercury's_Treads
if you notice under cost analysis, mercury treads are 94% gold efficient (aka not cost efficient unless you factor in tenacity, who's value perpetually changes based on circumstance). And a lot of players have been forgoing these boots since you can already nab a good deal of tenacity off of the defensive tree (which is why any tank buys merc treads in the first place, you run a quick check when buying to see what cc the enemy has, then you decide if it's worth it).
and Maw of Malmortius actually sits at 99% cost efficiency until the passive is popped (which means that if you aren't taking magic damage, then the MR stat isn't being used and it rapidly drops below 100% efficiency). The thing about maw though, is it's not an item that anyone would rush first to maximize damage output. It's a reactive item for when you anticipate the enemy will be bursting you down with heavy magic damage. Most assassins would much rather be grabbing the almighty longsword, then building them into youmous or the new duskblade (or hydra). And most ADCS would rather build into IE/ER and then follow up with their attackspeed item.
And AD champions have to care about what items they buy as well:
after core AD items (AD champions do care about dishing out their damage of course), you need to build reactively to your opponent (otherwise you lose one teamfight and then deathtimers and then there goes your towers and there goes your nexus):
Heavy armor? go get some pen
Zed? go grab a QSS
Leblanc? go grab a hexdrinker
juggernaut fed? frozen mallet works wonders against garen
poke? you might be able to heal back some of that caitlyn headshot with lifesteal
basically, what I'm saying is that there are decisions to be made with AD. It's not just build pure damage. An ADC is expected to deal damage- which is why their core item build (usually big ad item+big attackspeed+crits item) is there. A similar case exists with mages- they don't blanket cover all stats in their builds. Usually they buy their core items (archangels+roa, athenes/morello/frostqueens (is this still a thing? is spooky ghosts mid still a thing?), liandries/rylais, ludens/abyssal scepter), then they too must decide if they want to invest in more ap with a deathcap or perhaps go for a void staff or hourglass or rylais or ludens or abysal scepter.
And finally, you're arguing that APs need to itemize towards every stat that affects their to contribute...
and then you say that ADs need to itemize both AD, attackspeed and critical strike chance...
which just means that ADs need to use every autoattack based stat possible as well to increase their DPS....
just like what you argued with casters using every spell-based stat (except for spellvamp, which is only purchased on champions who care about sustain)
perhaps you were trying to say that mages can contribute more than just damage, they contribute utility such as CC, buffs and support (in the form of healing/shielding).
So what? Is your argument that ADs are dealing too much damage? because you just said that ADs are just damage (like that's all they do, that's all they have). So... you want AP to provide more damage than AD on top of all of the utility? You want a playstyle entirely dedicated to dealing damage to not be the top tier damage dealer? That's terrible! That's what leads to imbalance. What's next, you want thresh to outheal soraka now? It's that kind of logic that leads to Nautilus/malphite/Poppy 2.0 where we have a champion that provides immense cc, tankiness and as of recently 100-0 damage. A champion that generalises in everything without weakness of any type-but ore importantly, one who outclasses everyone at their jobs. Why pick a tank like sion who gives cc and tankiness when you could pick naut? And after your issue with AD and AP: why pick any AD champion if AP does more damage while providing utility?
Perhaps you aren't saying any of that, in which case what are you trying to say then? Because you have a lot of incorrect statements and what sounds like a lot of frustration otherwise.
, all adcs would prefer the former but sometimes must give it up for the latter
(% health damage, faster, scales better)
or 
or 
or 
+ 
? Want to explain to me how that's a fair deal for the AP side again?
heal up as much as an ADC with any completed lifesteal item.
and one slot for boots, an AP build is 
, and