"Removing the Young Griff and Euron story-lines has crippled [HBO's Game of Thrones]"

Strong speculation on where George Martin was going in the books:

You know how Dorne, the Reach, and the Stormlands have all virtually disappeared from the plot? The reason is because the show-writers have had no clue what to do with those regions. And why would they? With the removal of Aegon, there's a huge void where the drama in those areas should be. In the books, Aegon has already seized much of the Stormlands, and the Dornish will almost certainly join him once the whole Quentyn disaster comes out. Considering the tension between Cersei and the Tyrells, it seems possible that the Reach will also take up his banner.

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Suddenly, Tyrion doesn't have to have a lobotomy the second they reach Dragonstone. It also means that there can be actual consequences to Cersei's actions. In the show, her blowing up the Sept and killing hundreds of people has literally no negative effect for her, because there's no one else for the people to support. In the books, this could turn all of the common people to Aegon, while also meaning that Cersei can still remain in control of King's Landing long enough to execute her wildfire plot or remain a threat for later on.

Speaking of its effect on Dany's advisers, the lack of Young Griff in the show has completely destroyed the entire character of Varys. In the books, its clear that Varys stated objective to serve the realm is BS, or at least isn't the whole story. He talks about serving the realm, but he supported the Mad King to disinherit Rhaegar in favor of the already crazy-seeming Viserys. He says he wants peace, but he tries to get the Dothraki to invade to prop up a mad, cruel king, and kills Kevan Lannister and Pycelle when they threaten to stabilize the kingdom.

In the books, we know that the actual objective is to put Aegon on the throne

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Moreover, Aegon's presence makes Dany's job infinitely harder, but in an organic and satisfactory way. Unlike Cersei, Aegon is young and charismatic and popular, someone who could rally the great houses and the common people to fight for him. That means that Dany has a genuine dilemma

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book-Euron serves a vital role in the story. He is the human agent of the apocalypse: we know that he is embarking on some plot to destroy the powers of the world so he can become a god. Credible theories postulate that he is a failed dreamer, a disastrous experiment by the three-eyed raven gone wrong, and that he is either working with the Others or is trying to unleash them for his own plans.

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In the show, they had to have the wight-stealing plot north of the Wall so that the Night King could gain a dragon and invade the Seven Kingdoms. But in the books, the person who will most likely A) steal a dragon and B) bring down the Wall is Euron. With Dragonbinder, he can steal away Viserion to make his mad dreams a reality. The whole storyline with Jon and Tyrion acting like idiots to support this wight hunt, and Dany losing a dragon for no reason is suddenly gone, just like that. In the show, Dany and Jon and Tyrion are responsible for the Others invading Westeros -- if they'd never gone north, the Night King would never get a dragon. With Euron's story intact, the Wall falling is truly due to something none of them could predict or plan for.

Euron's idiotic, annoying character? Gone. Say hello to the twisted, pirate wizard megalomaniac with a god complex, someone who is genuinely threatening and dangerous. Rhaegal dying to a ballistae ambush from ships sailing in open sea, even though that's unsatisfying and makes zero sense? Gone. If Dany loses a dragon to Euron, it'll be because of the dragon horn, a genuine magic device that would have been built up for maybe 3 seasons in the show, only to be unleashed now.