Since I’m probably not ever going to write an in-depth meta about this topic, let me give you some reasons why Dany burning KL would, indeed, be unbearable if it doesn’t meet the requirements that Júlia mentioned in her tags. This comes from conversations with her (@rainhadaenerys) and Navya (@yendany).
It makes both Dany and Cersei, the main female rulers of the book series, look comparatively worse than both Jon and Tyrion. (And yeah, it definitely matters that they would look worse than these two and no, having other secondary male rulers fail and other secondary female rulers gain power wouldn’t amend this)
Also, Arianne Martell often dies in this speculation too.
Having Dany burn King’s Landing is repetitive. What else will she learn from this that she hasn’t already? That she has “been more khal than queen” and that she is the “mother of monsters”? Oops, been there, done that.
When Dany worked to ensure that there wouldn’t be any collateral damage (from the end of ASOS until ADWD), there was a lot of collateral damage. If/when Dany becomes less careful about collateral damage, there will be a lot of irreversible collateral damage again? Damned if she does, damned if she doesn’t again? There’s a sense of inevitability that might be what GRRM is going for, but it would still be unfulfilling considering all the development Dany received as a leader and a ruler during these books.
But that sense of inevitability is deliberate, isn’t it … because then Dany will sacrifice herself to save the world! Isn’t this so amazing? Isn’t her destiny so much greater? Uh, nope. Not really. As @khaleesirin laid out before, this ending would greatly diminish the impact of Dany’s Slaver’s Bay arc by making it subservient to the resolution with the Others. It would diminish Dany’s storyline for the sake of Jon’s, which I can never be okay with.
Besides, Dany has been established as a superior counterpart to Stannis in pretty much all the ways, from her political agenda to the fact that she is the true Azor Ahai. Why does she need to fuck up in King’s Landing like he did only so she can then realize the “true” enemy? Again, it’s too repetitive, which is why it’s fanon for now. (More on that below)
In any case, the books don’t necessarily align with this speculation. As @rainhadaenerys already pointed out here, the prophecies from the HotU align with the phases of Dany’s arc. In particular, we have the fire “for life” (dragons), the fire “for death” (whatever happens in Slaver’s Bay) and then the fire “to love”. Some have argued that the fire “to love” means that Dany will sacrifice herself for humanity, but that seems unlikely. If it were meant to be a sacrifice, it would be “fire for love”, but nope, it’s fire to love". This clear distinction, alongside the fact that the Walls of Qarth indicate that the love is romantic and sexual, should make us pause. If the third phase of Dany’s story is characterized by the “fire to love”, I don’t see how having her burn KL fits there.
Then there is also the fact the show may have indicated that Dany isn’t involved in the burning of KL at all… Well, to be fair, what happened on the show can validate whatever it is that you want to happen in the books. If you want Dany to be the final villain, the show may validated your nonsensical and misogynistic theories (…but let’s face it, it actually didn’t because GRRM is a good writer). If you want Dany to be a self-sacrificing hero, the show may have validated it by switching the order of the events for whatever reason. If you don’t want Dany to burn KL (and I certainly don’t), well, the show had her snap due to bells when that’s actually Jon Connington’s trauma. Since I’m an optimistic bitch, I’m going with the third option that he and fAegon will burn KL (better it be them than the character I’ve been following for five books).
Then there are also the petty reasons why Dany fans shouldn’t be okay with this theory that I’ll mention anyway because I’m exhausted.
First … Look, there are some meta writers who defend this speculation and like Dany. I like them too.
But there are others who:
Think that she must acknowledge and pay for her father’s sins (as if she wasn’t carving her own path long ago).
Think that Robert’s Rebellion is as justified as Dany’s crusade in Slaver’s Bay (it absolutely fucking isn’t).
Think that she’d be tempted to ally herself with that disgusting rapist to build a new Valyria, which is out of character and would warrant criticism if it were to happen (But these people would rather talk about how the writing of Obara’s mother is sexist rather than think about the negative implications in their speculation of Dany’s future or even, say, Arianne’s).
Think it would be awesome if Dany were dispatched in favor of minor characters that we still don’t know almost anything about like Willas and Garlan Tyrell and Sarella Sand or in favor of the Starks, whose restoration is certainly not more righteous than a Targaryen one. Yup, destroying the Iron Throne is COMPLETELY NECESSARY! (Even if they also acknowledge that rulers like Jaehaerys and Alysanne improved the realm.) But the feudalist system that subjugates the peasants by virtue of existence? Nah, that one can stay. Only Dany needs to go.
Being 100% okay with the theory that Dany burns KL and/or dies saving the world means being okay with the opinions above too. Let’s not pretend that this speculation isn’t pro-Stark/fAegon/Stannis (before Shireen’s burning was revealed in season five, many of these people used to hope that Dany would die and that Stannis would have the chance to be Lord Commander; convenient how destroying the IT is necessary, but then all your faves live to rebuild the world, huh?) at its core.
(Also, the idea that Dany will sacrifice herself and die a hero after burning KL is mostly popular on Tumblr; elsewhere she’ll be a tragic Shakespearean hero, which is even worse for having the influence of BryndenBFish and being unabashedly pro-Stark. I wasn’t going to use names, but I’m tired of seeing these theories floating around as if they weren’t malicious. If you think these events are probable, you should be criticizing the hell out of them, not praising them as satisfying outcomes.)