Not only because of the age, but I think they would have nrought a tiny gravitas to the thing. And with a little squinting Blanchett COULD look like...
Honestly, it was a waste not to use Glob in Swords, especially since Brisson was indeed using him and building him up. We got a better X-teens scene...
Didn't Xavier and Magneto chose to do so by themselves ?
Also depending on who was on the council (i don't remember), but Destiny and Shaw would not...