Peter came to prominence in the general public's eye a little more than a decade ago, while Batman and Superman's alter egos have been a part of popular culture for over 50 years.
Think this makes Peter Parker even less replaceable and more iconic. The original Spiderman films were the equivalent of the Christopher Reeve superman films for many kids of my generation; it was the first big superhero film I'd ever seen and it certainly helped catapult the entire genre to it's current prominence (not to mention shape it).
Grant Gustin would have be so perfect
He really would have, it's a damn shame DC isn't using him in the films. Awesome and immediately likable actor.