I have just finished the game, got an ending where Ciri becomes a witcher, Emhyr wins and Geralt stays alone. Now I am doing my second play-through to get a canon (in my view) ending: Geralt with Yen, Ciri is Emperess. Let me share my thoughts why "Witcher" ending is not the best, not for Ciri, nor for Geralt.
1. Ciri is not a true witcher. Don't recall where (book or game), but Geralt once said she was never to be as good as him. No matter how much he was teaching her, shes only half-witcher, missing on a physical part. Remember the book scene where Geralt's life was saved only cause he had ability to slow-down body functions being mortally wounded? Ciri does not have that. She cant use elixirs. This is bound to end bad, with just a single wound, which witchers usually get plenty.
2. I was really surprised how unfinished was ending "Ciri witcher - Emhyr lives". Emhyr was searching relentlessly for her for 20 years. No way he would trust Geralt on his word that Ciri was dead. And that phrase about Ciri becoming famous? She would draw Emhyr attention at once, becoming a hunted prey once again. And Geralt would never rest easy with Yen/Triss as Emhyr would never forgive him his lie until Geralt is dead.
3. Even if Emhyr leaves Ciri alone (which is very unlikely) or Emhyr dies, lets not forget that Ciri is a rightful heir to the throne. From the book we know how exiled Emhyr reclaimed the throne from usurper. Those planning to take over after Emhyr wont let Ciri live cause of that. Like in a book when kings of the North decided that Ciri has to die, just in case.
So looks like "Ciri witcher" will lead to Ciri's death in any case, from assassins or monsters, and Geralt's as well in case Emhyr lives. Even if they somehow manage to avoid it, this means a life of constant running and hiding, far from that idyllic life we are being shown in the ending. Ofc it is much better then ME3 ending, but still far from well-thought.
A very astute observation. I completely agree with everything you wrote. The "Witcher(ess) ending" is not only the most sappy, self-indulgent and plain logical inconsistent out of the three possible conclusions, but, more importantly, it doesn't make sense from a thematic standpoint and undermines Ciri's arc of self-actualization and character growth and opts instead for a continious state of arrested development.
Whereas the "Empress ending" doubles down on Ciri becoming a self-sufficient, mature person who takes control of her potential as both an inherently powerful wielder of magic as well as royal heiress of two mythical lineages and ultimately learns to, if not embrace, at least accept her specialness, the "Witcher(ess) ending" has Ciri, once again, shed (and run away from) responsibility - despite her clearly stating throughout the game how fed up she is with constantly being on the run - and taking up an occupation she is not only inadequately suited for (as comprehensively explained by Cromanin) but one, one can't help but feel, that is most likely only a temporary diversion for her.
I feel that Ciri wanting to be "a simple Witcher" is more an expression of her desire to escape from all the responsibility and peril (Emhyr, The Wild Hunt, Lodge of Sorceresses, etc.) than an actual wish to really follow in Geralt's footsteps. It's important to remember that by the time the final battle is waged, Ciri has clearly evolved and matured from the person that just wanted to be left alone and has finally taken control of her destiny - to the point that she leaves both her "foster" parents in the dark about her actual plans regarding her impending battle against the White Frost.
Another thing to consider is that Ciri would be severely ill-equipped as a Witcher not only due to her incomplete training and fragmentary knowledge but even more so because she's an impatient, hot-tempered person who is prone to righteous indignation. If there's one thing that is paramount to being successful (read: not immediately killed) in this line of employment it's patience, resourcefulness and restraint. All characteristics that Ciri somewhat lacks.
And while her strong moral standards are an enviable character trait from which whole nations and races might benefit when channeled in the correct way and under the right circumstances - say as a magnanimous and tolerant ruler of a powerful kingdom - it might very well spell doom for her in form of a premature death if these character attributes were to come to the forefront in a less than desireable situation. Geralt knows a thing or two about that. After all, there's a reason why Witchers tend to be neutral, or at least feign neutrality. It has, to some extent, to do with self-preservation. Also, job security. No one hires a judgemental, socially concerned and tradition-violating Witcher.
To be honest, I have the sneaking suspicion that the reason why so many people here prefer the Witcher(ess) ending and consider it (head)canon is simply that it allows them to conceive of further imaginary adventures featuring an ass-kicking, monster-slaying Ciri that take place after Witcher 3: Wild Hunt ends, something that both other endings deny them. And yes, Ciri ascending the throne as Nilfgaard's empress is a denouement tinged with both hope and sadness, and thus perfectly in line with Sapkowski's prose. And as the old saying goes: Good storytelling doesn't give you what you want, it gives you what you need!