On the main thread about upcoming models, the EOPs of the current gen are listed as:
F12 6er - 10/18
F13 6er - 10/18
F06 6GC - 02/19
Note this doesn't say that the M6 or any other specific model will continue to that point, only that they will be making cars based on those chassis until then. So there is a chance they could stop the M6 earlier but continue the 640i, 640d and 650i depending on demand but I think that is unlikely.
The placement of the M6 in the market is unique and is probably the problem it has. While there has been an E Coupe it was never available as an AMG and was actually based on the C class until the new generation just coming out which is rumoured to be coming as an AMG E63 Coupe.
Everything else that people compare it to is either a class above in size and luxury (while being a hell of a lot slower) such as the S Coupe, Bentley Continental or similar cars. Or it gets compared to 2 seaters such as various 911s, Corvette, R8, AMG GT, F Type and naturally against those it will be considered heavy and not as capable on the track, but on the other side of the coin it is more comfortable and a better GT than those cars.
Potentially the M6 could evolve into something based on the 4 series coupe chassis but with options for a V8 which would result in so many fun arguments against the M3 and M4 crew on here
Or maybe it does become the 8 series and is designed to compete against the S coupe.