Peoples isn't improper grammar. It's more used to refer to specific groups of people rather than just a generic gathering of people.
Like, you could refer to "The peoples of South Africa" and be talking about the various ethnic groups represented there.
I mean, there's no specific instance I can think of where it's grammatically necessary to use it, but whatever. You CAN use it.
--
"Stay with me until the end. Please."
"Not until the end. Always."