I would argue that it was an anomaly for Combat to have such low single target DPS with WoD.. In the past Combat has been above average to very good or even the pinnacle of single target sustained DPS.
Multistrike was a big factor in Combat's single target DPS being tuned so low in WoD and with the removal of it I expect Outlaw single target sustain to increase. Especially with Kspree a talent choice now which means they will tune Outlaw around Curse of DreadBlades and Adrenaline Rush primarily now.
Sub tuning could end up making it a stronger sustained DPS spec in Legion, but with the charge system I suppose it depends on player skill more so. With Outlaw it is more about fishing for RNG True Bearing procs. And Assassination is a darkhorse to possibly surprise for PVE as well.
Damage tuning is definitely around the corner and it will be interesting to see if Sub stays true as the ultimate single target spec (seems unlikely but you never know).