Look, my overall opinion is that music[k] always has and will be, changing, hence, new ideas new ways and new, or altered, instruments and as such, score writing.
Way back, the predecessors to the modern guitar, like the luth, had less than six strings.
EDIT: Was thinking of another instrument - the luth could have many many strings; bad choice..
What if the guitar, as we know it, had been made with five strings - and then someone had the nerve to add a
sixth string? Same thing as with today's multi-stringers..
The early double bass had three strings; the fourth was added, and today we can 5-string buy carbon DB's.
Will anyone oppose me if I start playing violin again, just now on a 5-string carbon fiddle? - hint: that extra string would be for additional lows, and, apart from learning to play beautiful, I'd also learn to rasp the bow staccato for producing low chops to go with .. tada .. djent!
Anyone heard of Wesley Belmont? No? One of his arrangement is titled "Do you even hyperdjent, bro?"
Here's an excellent example of making 9-strings work right - if you can get yourself to dig his style
The reason his works are crisp with attack down there is the use of a 30" Agile, which allows using not-too-fat strings.
(BTW, this arrangement would be super sexy with an added fretless bass)