I’ve talked to some folk who work in TV about just this.
The upshot is that knowing the length of run with enough time creates better episodes whether the run is short or long.
The problem is something getting big and an emergency demand for more episodes after a season has already been planned out. (or that episodes need to be cut for a reduced run).
As for the BBC, we are still very much waiting for the clock to run out on Casualty, Holby City, EastEnders, silent witness…
I’d also argue that even despite a shorter episode length, Sherlock still managed to overstay its welcome.