Why is protecting the U.S president important ?
I mean really, the president is pretty expendable and easily replaced in my personal opinion.
Furthermore most presidents don't follow up on their campaign promises so as their term progresses their approval rating plummets so wouldn't it make sense to take away those armed guards once the approval rating drops below a certain point thus the fear of such a thing happening would help keep them honest and doing everything they could to remain popular with the people.