Aegon's prophecy was the key concept of House of the Dragon season 1. It was what motivated the characters and drove the whole narrative. Indeed, you could argue that it was (and is) the premise on which the whole franchise is based. So how was it so wrong?
Of course, we know that Targaryen prophecies have a habit of coming to fruition. And had House of the Dragon been released prior to Game of Thrones, this may have all been forgotten by the time the (presumably much better) ending came about.
But the truth is that we know what happens at the end of GoT.
When the White Walkers arrived, Westeros was far from united in standing against them. No warriors were sent from the Westerlands, the Reach, the Iron Islands, or Dorne. That's more than half of the Seven Kingdoms not showing up!
So, the Iron Throne was not sat upon by "A king or queen strong enough to unite the realm". And to be fair, there was no hereditary monarch or Iron Throne by the end of Game of Thrones. Finally, of course, it was a Stark that killed the Night King.
Which means Aegon got it all pretty much wrong.
Now, this does not necessarily mean that the whole prophecy storyline is wasted. Although it does bring about the question of why so many lives were lost in pursuit of a perceived belief that would eventually turn out to be untrue (a bit like most wars).
What it does mean, though, is that the showrunners have got some explaining to do. There are plenty more series of House of the Dragon to come and many more years of Westerosi history yet to cover. So, time is on the side of the writers. And the ball is very much in their court.
Failure to deal with this burning issue at any point as the story of the Targaryens unfolds will leave a sour taste in the mouth. Of course, this may simply be a legacy of the dreadful ending to Game of Thrones. But hey, that's been done now, and it appears there's no going back. So, the responsibility to repair what is a fundamental part of the storyline that runs through and connects the two shows lies squarely with those charged with writing future seasons of House of the Dragon.