The entire movie was mediocre IMO. As for the movie's "tremendous success". The critical response was decidedly mixed; 47% positive on Rottentomatoes. It was a box office success but as superhero tent poles go it was nothing special; Budget of $178 million and world wide box office of $544 million.
For comparison; Wonder Woman Budget $149 million and world wide box office $822 million.
That already shows it wasn't a tremendous success.
Nevermind that it dropped over $200 million from the previous movie. It was the least successful X-Men movie at the time in the US.
It underperformed as it was expected to make $100 million in its first 4 days but instead only did $80 million. Then in a series that was already very front loaded to begin with, Apocalypse was far more front loaded still meaning it didn't even finish with double it's opening weekend.
In fact it was nowhere to be seen in Deadlines Top 20 most profitable movies of the year where the #20 movie had a profit of $52 million. It wasn't even mentioned with the three honourable mentions either where the lowest was $37 million.
Then of course people didn't much care for it at all, leading to poor reviews and so so audience reception which then meant that nobody really gave much of a shit about Dark Phoenix from the beginning.