The 717's are apparently long of tooth, so they probably need to go sooner rather than later.

But the 321's can hang out for quite a while, I don't think they're going to cut off their nose to spite their face. They can keep the 321's as long as they need the lift. If there was a big downturn, like another covid or big recession they'd get parked on Jan 1st. If things keep humming along they might last 15 years, or however long it takes BCA to deliver the hulls they need with the desired fuselage stretch.