Ultimate Pete could not go back to being spider-man. His identity was already outed, and he was legally dead. Assuming the original intention of Pete leaving and maybe coming back to NYC much later on had the Ultimate line continued, he likely would would have become Ultimate Scarlet Spider.
The original premise of Miles in 616 is that Parker would be dealing with 'world level' threats as CEO of Parker Industries, while Miles remained the 'friendly neighborhood' variety Dealing with NYC. They couldn't make CEO Peter interesting so he got another downgrade back to street level which is were we're sitting at right now with the city being split between the 2 heroes.
He's a Legacy Character and Marvel's most successful one at that. He is Spider-man. Cope.