From a storytelling angle, I think it needed to happen sooner. After you read about 3000 pages of characters you come to really connect to and care about, and watching many of them die, and in general get the overall feeling that the main storyline was moving towards the end game....only to then directly jump into an introduction of a whole new cast of characters...that's more than a little annoying. It's equivalent to if after the battle of Helm's Deep Tolkien left all the fellowship characters for half a book to introduce a whole new cast of random elves and dwarves to follow.
There was always going to be at least three more novels of the same length following A Storm of Swords. Honestly I'm not sure how you thought that the series was approaching the endgame with that in mind, certainly a midway point had been crossed, but the series wasnt ever intended to be a quadrilogy and each novel introduced new POVs to follow. Clash of Kings had Davos and Theon, A Storm of Swords had Jaime. Pretty much all of the Ironborn had been introduced as characters in Clash, just like how Jaime was introduced in AGOT, and even going right back to George's earliest planning, the Kingsmoot was always going to open the potential 4th novel after the aftermath of Storm. The idea has always been that the first three novels dealt with the War of Five Kings, and the last three dealt with the more overt supernatural threats. Given that Euron was basically foreshadowed throughout the first trilogy and then made his appearance right after, I don't think there's anything wrong with that, especially since the White Walkers themselves weren't used a lot in the first three novels either.
On top of that, many of the iron born felt like rehashes of characters we've already seen. Enron-creepy sadistic brother who's manipulating all the iron born to his own ends, and he might be somewhat magical. He's Ramsay mixed with Littlefinger and a little magic thrown in. Vicatrion-feerless warrior who plunders and pillages...Kahl Drogo on a boat. Etc.
See this is kind of a weird view, because on an aesthetic or thematic level, Euron is nothing like Littlefinger. And he's only akin to Ramsay in the sense that he's a psychopath who tortures people, but even then, Ramsay is not the character of the show. He's dumb muscle in the books, a thug who Theon was foolish enough to fall for. Everything about Euron screams Lovecraft, he's walking apocalypse. Likewise, Victarion is only like Drogo, in as much as he and Drogo are both based on Conan the Cimmerian. Drogo was basically just Dany's love interest whom we only saw via their love scenes. Victarion is a POV character whose thoughts and emotions are open to the reader, and his arc is unique as a villain who views himself as a hero.
Added to that, if Euron's arc essentially becomes "bad guy sails to Danny, tries to control Danny, get's fried by Danny and she takes his ships" I'll be even more annoyed he was given so much book time this late in the game.
Dany has no need to take Euron's ships, because Euron isn't bringing her ships. Euron is in the Reach about to unleash the Kraken on the Redwyne fleet.
Victarion is the one who is sailing to Dany, and by the looks of it that crazy bastard might actually be in love with her and join her side. The newest chapter basically flat out shows us that this guy is
very apocalyptic and factors into the White Walkers in some real way. Him being simple a road bump for Dany is massively underestimating the story, especially when all factors point to him factoring into the endgame.
You might not like the Ironborn, which is fair because they're not everyone's cup of tea, but just because you don't like them doesn't mean that they're unimportant to the narrative..