But they didn’t choose to end it. In fact, Swampy didn’t want to admit that they were cancelled back in 2014. He and other crew members claimed they were on “indefinite hiatus.”
While I’m sure Dan & Swampy we’re happy to try something new with Milo Murphy’s Law, its second season as well as the upcoming PnF Disney+ movie are proof enough that their hearts were always set on making more Phineas and Ferb.
TL;DR: Dan & Swampy didn’t choose to end the show. They were cancelled and forced to write a series finale.