As has been alluded to by others - they tried to get one in canon under Karl Franz, but the other Electors voted against it because they assumed (probably correctly) that the Colleges' Elector would almost always vote with the Emperor, which would have centralized power more than they were comfortable with since the Emperor already had something of a voting bloc behind him. And of course all 3 Sigmarite Electors were dead-set against.
More generally, regarding the "the Colleges are too powerful to plausibly be excised" take, I'd be wary of extrapolating too far from the aftermath of Dieter's clusterfuckery. Most notably, the rest of the Empire never actually really tried to get rid of the Colleges then. The Witch Hunters had a go (and those who went after the Colleges' main campuses got exactly what they had coming), but the Colleges never actually had to contend against the full power of even one province, much less the Empire as a whole. If the Colleges got busted violating the Articles, that could potentially change, as at that point it looks less like "the Grand Theogonist has a bee in his bonnet" and more like the Colleges could actually be an active threat. It should be noted that while the Night of a Thousand Arcane Duels was a consummate shitshow, it didn't actually involve breaches of the Articles per se AFAIK.
I never said the wizards would win. I said they would not lose. Big difference.
The wizards have a huge tradition of not losing, seeing as they had been hiding and even exercising influence before Teclis even organised them. The Hedgefolk still keep to those traditions. And the wizards were weaker magically and politically back then, plus much less united. They can now hide better, and punch back better, as has been shown.
Also, while I agree that the wizards would lose if they have to fight a war of attrition or a war of firepower with the Empire, I find that info to be completely immaterial. Yes, it is likely true, even if the Empire would have to expend a ton of resources. Heck, it is possible that, as you say, a province would be enough, (most) wizards are squishy even if they can artillerate, a huge army would be a problem. But that is not how it works, its never how this works, I am not talking about a "who would win" in a flat battlefield here, I am discussing realpolitik.
The wizards won't fight the Empire on an open field, it won't be merely a subject of the Empire spending enough resources, the wizards will be hiding, assasinating, spending political favour, crashing economies, calling favours, sabotaging, using guerilla warfare etc. Or they'd be doing nothing but hiding until another Emperor comes in, and good luck uncovering them, half the colleges can just phase out. And the Empire, or even a province, won't be able to just bring the (war)hammer down on their heads. Even assuming the wizards lack political connections completely and are fully insular, (which is not how it works), the Empire is always besieged by a hundred threats, plus bickering with itself. Nothing sort of a chaos invasion will bring its full armies to march, sometimes not even that. Even EC cannot bring their armies to bear, what with beastmen, vampires, orcs and... other ECs waiting for a moment of weakness. Nobody will want to expend too much resources going after the wizards, and some would hate to lose wizard support. To be a power pole nobody but a complete idiot would think of getting rid of (or someone dumber than a complete idiot, such as Dieter IV) you do not have to be the kind of entity that would win, you'd just have to be the kind of entity that it'd really, really hurt to lose as an ally/ gain as an enemy.
To state it in a different way, most of your arguments would arguably apply to any single Elector Count province. Let's assume an ideal scenario when the other ECs would not feel threatened by such a thing and support the obliteration and conquest of the province, for some reason. The Empire as a whole would steamroll the countship if it used all its power to and, if we are talking about a weaker one, a single of the strongest provinces could beat them. But that is not the subject at hand, is it? The subject at hand is, they can fortify and draw things out, the Empire would be losing a big part of their powerbase, politics would happen, human carrion smelling weakness would gather around those spending the most and chaos and orcs would be left to their own devices.
The full army can't be gathered, the full political might cannot be exerted, and the Empire consists of many conflicting power blocs, of which wizards are but one. A scenario of Empire vs wizards is as unlikely as a scenario of Empire vs Stirland. And yes, it can happen, Stirland could be conquered by vampires, and a second Dieter IV could come to power. But it could be very painful , and unless the threat was existential, it would more likely than not result in a long stalemate and another enemy to the pile the Empire has to contend with.
Edit: which is not to say that they are impervious, losing legitimacy by being considered a corrupting influence would still mean that people would prefer amputation and risk of death to certain death by gangrene, just that the realpolitik does most of the heavy lifting for keeping them hard to assault.
