In fact, Martin Freeman and Benedict Cumberbatch’s busy careers factor into Season 5’s current position beyond the horizon.
Around the time Season 4 was released, co-creator Mark Gatiss mentioned that more episodes would be hard to pull off, with Benedict Cumberbatch’s MCU role as Doctor Strange keeping his future erratic.
With Freeman staying equally busy both in and out of the Marvel universe, it’s not going to get any easier to coordinate these actors’ schedules, and even attempting to do so might cause one or both to turn down a role they’d been interested in, which might be why Freeman told DenofGeek he’s glad development towards Season 5 of Sherlock isn’t chugging along.