The ending was just fine. There is such a thing as leaving the end of your book a bit ambiguous so the readers can fill in the blanks for themselves. And as others have said, Randall Flagg is immortal. Why would he die at the end of this book? This article comes off as a bit uneducated.

Yugioh GX was subpar. A lot of times it felt pointless, like a lot the time the story felt like it was just going on with no end goal in mind. The first big bad kinda just came out of nowhere and was defeated just as quickly as he appeared. Then the Sartorius arc, while good in theory and concept, took way too long to get to where it was going and became way too convoluted and boring. And Jaden not having to rely on magic like Yugi did? Seriously? Jaden literally sees monster spirits that help him duel. And he also has insane luck when it comes to dueling and he relies on that luck to win. Whether you want to count luck as magic or not, there's no denying there something unnatural about the way he wins almost all of his duel.

Jaden is also very arrogant and overconfident. It's very hard to get behind a character like that, especially after having a character like Yugi, who was so selfless and easy to root for.

Simply put, Yugioh GX doesn't come close to the OG Yugioh in any way.