We have to remember that Dan & Swampy never wanted to end Phineas and Ferb in the first place. A recent video chat interview Dan took part in put more light on the subject.
It got to a point where Disney was holding back episodes for so long and the PnF crew was so far ahead in production that there was nothing for the writers and artists to actually work on. IIRC, Disney "wasn't ready to greenlight another season just yet," but they'd instead keep throwing hour-long specials at the crew to keep everybody busy and so there wouldn't be any need to let people go on indefinite hiatus. After enough times, Dan & Swampy got tired of this waiting and feared that they eventually WOULD have to let everybody go, so they made a compromise with Disney to just let them close out the show with one final "finale special."
Dan & Swampy would've loved to make more Phineas and Ferb, but they had no choice but to demand to cut it short. That doesn't condone their dumb decision to make a second show whose ultimate purpose was to bring back the PnF cast, but this info at least adds some context to their actions with MML.