I will start off by just pointing out that your entire assumption that mages are weak end game is entirely based upon the first 25% of the game. Hardly a fair point to make.
To be more specific, magic damage dealers are weak and because of that, magic "debuff"ers are also weak.
What? No. Right now mages destroy most fights. I am not sure what you are talking about for debuffers? Necromancy has decaying touch and shackles of pain. They can also cripple everything in a large aoe. They can slow everyone and reduce their ap regen that bypasses magic and physical armor. They apply dots, CC, charm, their utility is insane. I just re read your post and see you acknowledge necromancy's strength.
The most noticeable difference is none of their abilities are affected by weapon damage. This by itself makes their damage output significantly worse and keeps them from applying crucial debuffs.
I will agree with your point about scaling based off of weapon damage. I think they scale better off of stats, but I could be wrong about that. However, I am guessing you have never used a poison mainhand and fire offhand with wands... That combo right there can do insane damage. It causes explosions, takes a %based chunk of an enemies HP AND causes a fire surface. It straight rips through magic armor like it wasn't even there ALL FOR 2 AP.
There's also the factor of dealing with longer cooldowns that their physical counterparts and having less mobility options, and since positioning is so important to having a succesful fight this gives physical classes an upper hand
The cooldowns are longer in some cases. But are usually on par with the physical abilities. Also mages are far stronger in terms of positioning than warriors/rogues, because they don't have to move around the battlefield. Warriors are rogues are actually at a disadvantage than mages because they have to be so close to their enemies that they are in range of friendly fire, and enemies no longer have to waste AP moving to them to attack.
The other big factor is the prevalence of elemental resistances and how common they are.
If you have loremaster you can see that a good majority of the physical type enemies have high physical resistances and lower elemental resistances. I feel this is a moot point.
While Teleport is overpowered, this doesn't compare to some of the broken stuff that physical classes have. Adrenaline gives 2 free ap immediately, Skin Graft resets all of your cooldowns (too bad mages don't have this)
This point is only valid if you assume that a mage can't/won't put a point into polymorph or scoundrel. If you build a strong mage you will absolutely put a point into both of those, to have access to those schools. Nothing in the game prevents you from choosing to do this so I am not sure why you feel like mages can diversify.
Snipe doubles you'r damage while sneaking, Mortal Blow does x5 damage while sneaking, Chameleon Cloak is a starting ability that gives invisibility, Overpower gets rid of the enemy's physical armor, etc...
Again you are lumping a utility based skill set into a physical only club. Mages can choose this. If you say that a full magic build can't have access to this line of utility skills then you have to say that warriors and rogues can't either. Which doesn't make sense at all. I will absolutely agree with you that mortal blow is flat OP. Like damn... However, mages can make cursed fire, acid surfaces, cursed electrified steam clouds (like seriously OP). They can perma stun/kill entire groups of enemies with Acid Spores + any fire spell.
Basically, the only usefulness of magic is on a Hybrid build and any Magic only builds are not viable.
I will wholeheartedly(sp?) disagree with you on this.
I say this not to sound condescending so please please please don't take it that way. But I feel like you are not building a mage in an efficient way or aren't setting up their combos and CC very well. I think you have a few valid points but you make a lot of untrue assumptions.