Steven Moffat leaves Doctor Who and is replaced by.....

[jwl]

The other thing is that with Who going in hiatus every so often, and only 12 or so episodes a year, the current shooting schedule is probably less onerous than old-style Who with its multi-episode serials.

Really, though, it's up to Capaldi whether he'd like to go more than the typical three series or not.

Not really. The modern style of shooting television is with multiple takes in lots of different locations, the same wasn't true in classic. And in the tom baker era, it was ~26 25 minute episodes, which is the equivalent of ~13 50 minute episodes. The first doctor had more like 40 episodes, but he went on holiday during serials and only did one take.