The first few posts in this thread are referencing things like originality and creativity. I don't believe a vocal style can box a band in to being original or stagnant.
I can point you now just 2 guys that are doing their thing in singing style that makes things original and yes their voices are part of the originality. You have Mike patton and Devin townsend. As for Mike patton, the man have done almost it all, to most fuck up growls to most insane clean vocals, so he is quite a perfect example, how to use the voice with originality and bringing originality and creativity to the context of the music genre he does.
You rely your argument heavily on shitty clean singers, the thing you dont seem to understand it´s that I am not talking about mediocre people in their business and I can complety agree with you that shitty singers doing cleans ruin music even much more faster than growlers, because the writting process for both is also different. There are probably a lot of clean singers in modern metal music and most of them suck donkey balls because they cant sing for life, and in this case, yes I completly agree with you, I rather have a awesome growler than a weak clean singer.
So I am pointing people that excels in their shit, not the mediocre ones. Now I can grab the example of people doing their shit in their room, everyone now can throw albuns out there, and this is one of reasons why the quality decresead because there a lot of shit being done everyday, too much quantity over quality, so anyone can pass as a clean singer, yes it´s true but as a good one? there are few. I care for shitty singers like I care for shitty growlers, that is nothing. But that doesnt mean that I cant ask for people that knows how to do their shit. Because I will say to you that are much more but really much more bad growlers out there that shitty singers. If you want to sing, you have to know how and you must show ability to do it. Now the growler isnt the same thing, as for the good growler yes but there are so many bands that cant get away with mediocre growlers and people dont care much about it, now the clean singing, if you dont know how to do it you will make fool of yourself but as for the shitty growler the things arent the same, people dont think much of it, it´s a part of the agressive sound even if he does it right or not.
So are this part of the state of metal in 2016, of course it is, singers have relation with the state of metal or arent they part of bands and writting process? Of course their are. A good or a bad growler is always part of the creativy and of the originality of a style. You pointed the metallica example, now I can point the Tesseract example, isnt their singer part of their creativy and originality, of course it is and a lot. You may not like it but that´s another question and I can make a point here also. if tesseract had a growler? Would be the same? even a good one? I think you know well the answer. Even with a good growler, if you can put there a good singer it will make a world of difference. Lets suppose that they had a awesome growler, even with it would be much more hard to break and make the difference from all the bands who have growls.
I can point even another example, part of the sucess of opeth is the clean singing and gives room for another kind of writting in the guitar instead of being limited to do death metal riffs all the way. He was also a good example of a great singer and great growler, as for the growler, unfortunately he was, isnt anymore. But those examples can support my view of things.