The sequence of Quicksilver rescuing Magneto from the Pentagon was one of the most memorable moments of X-Men: Days of Future Past, but it wasn’t always going to be like that. According to executive producer Simon Kinberg, originally Juggernaut was supposed to be the one that Charles Xavier, Hank McCoy and Logan would have recruited for the prison break. When Bryan Singer took over directing duties from X-Men: First Class director Matthew Vaughn, the script was adjusted to accommodated the silver-clad speedster, but now we know how the Juggernaut scene would have gone down had Vaughn stayed on.
During an interview on the Empire Online podcast, Vaughn went into full detail about how Juggernaut would have assisted in rescuing Magneto, and the scene in question actually took place at an entirely different location. Said Vaughn:
I had Juggernaut being thrown out of a plane at 20,000 feet and using his head to go down a hole to get to the bottom of [The White House]. I had Beast saying, ‘I have a plan,’ and him replying, ‘What’s the plan? What’s the plan?’, and they go higher and higher and Beast was going to go, ‘This is it!’ and chuck him out the window. Then he goes smashing through [The White House] ... and Magneto goes, ‘Who the fuck are you?’
As for the sequence moviegoers saw in the final product, Vaughn admitted that using Quicksilver was a better decision, calling it the "best scene in the movie" and saying his version "would have made the movie worse." Juggernaut’s only appearance in the X-Men films was in X-Men: The Last Stand. Unfortunately for him, his entire comic book history was discarded in favor of being Magneto’s muscle and delivering the infamous line, "I’m the Juggernaut, bitch!" Josh Helman was originally supposed to play the gigantic mutant in X-Men: Days of Future Past, but instead he was cast as William Stryker, Wolverine’s future adamantium supplier.
Aside from the age complications of using Juggernaut (if he’s actor Vinnie Jones’ age, he would only be eight years in 1973), having him be the one to break Magneto wouldn’t have been nearly as fun to watch. Between screwing with objects and people while moving at super speed and running through walls and other obstacles, the former is far more entertaining. Of course, now there’s the issue of why Charles and the gang just didn’t use Quicksilver to help them with everything else, but for the most part it all worked out. And since much of the X-Men film franchise has been reset thanks to Wolverine’s time travel shenanigans, there’s still a chance that we could see Juggernaut again someday.
It’s also worth noting that moving Magneto to the Pentagon was another wise choice. Keeping him underneath the President’s residence is just begging for trouble.