Long before Dark Phoenix, in the summer of 2000, X-Men set the box office ablaze and had the sixth highest-grossing opening weekend ever at that time. That paired with strong reviews from critics and fans alike, changed superhero cinema forever.
When the original was released, it entered the game just after Batman & Robin killed the then-most lucrative big screen spandex saga and as Blade was pointing at comic book adaptations geared more toward niche audiences. But then, X-Men featured well cast characters (Patrick Stewart and Ian McKellen as Professor Xavier and Magneto, respectively, remain iconic), dynamic super-powered action, and a serious approach to the themes that have long made these Marvel mutants resonate. It paved the way for Spider-Man to swing and Batman to begin (again) on the big screen. And then, eight years later, Tony Stark met Nick Fury and everything changed once more.
Source: IGN.com The X-Men Paved the Way for the Marvel Cinematic Universe