I mean, I feel like Jodie will leave after one more Series. Her interviews all called out Series 12 as "it's not my final year" or something similar; It was said in a very odd way. Why would you call out your second Series as "not your last year"? Plus this is gobbling up her life/career, this year even moreso- The only time she could've snuck in non-Who work the whole industry shuts down. By the time filming Series 13 wraps at this rate, it will have been 5 years of her life. That's a LOT for an actor with little else being done those 5 years. Maybe if she was a lifelong fan, but she's established she wasn't that either. Plus, the people she came on board with (Walsh/Cole, possibly Gill) will be gone, so any camaraderie they've had filming is moot.
I'd wager Chibnall leaves with her. He's gotten a rough ride and whether you think it's fair or not, remember: He was that fan watching JNT bury himself under Who. Now he's the guy fandom at large is demonizing. If he rallies and leaves Who with Jodie on a high note, the old "Make 'em miss you", it could be good for him and the show. Were the show done as frequently as it was with Davies, I feel he'd more of an advantage. But it seems like 12 month breaks don't benefit the show and his production style on Broadchurch hints that may be his preference. It's years of his life too, and right now he'll be promoting that 3rd season at the point Moffat had bragging rights to global viewing expansion and the 50th anniversary. Davies had just wrapped Series 4 and was doing a victory lap. Regardless of what you think of their eras, you have to admit they had more to show for them at the point that, if I'm being kind, Chibnall can boast of...strong audience showings his first year, some narrative risk, and (some) bold/groundbreaking casting.
I really am optimistic, and feel that the show always has the potential to bounce from weakness with a "Caves of Androzani" style rebound. I'd love to see those two pull off that high, and then clear the decks for something new. (Ideally, a bold producer from outside the fan circle that's dominated Who. A new series Hinchcliffe, if you will.)