The 335i coupe already competes with and owns the G37 and A5. The M3 coupe competes with and again owns the S5.
The new E coupe is bigger and a new style, BMW does not need to compete with that car, with the coupes they have available already sell a heck of a lot more than benz.
While people would love a 5 coupe, it doesn't make sense, the 6 is the 5 coupe.