With people tuning seven-string guitars do Drop G or using 8 and nine-string guitars and the playing in "genres" like Djent becoming more and more rhythm focused I've been wondering about the bass players job in the band. I've heard Tosin Abbasi say that with his 8-string he already is in the bass register. So I was wondering from what tuning onward (or downward) you're actually making a bass player obsolete.
Normal bass is a full octave below a guitar. So if you're playing drop G on a 7 or 8 string, you're pretty close to the bottom of a standard 4 string bass. A 5 string bass will have a low B. So the simplest answer is that they're already there.
However, that really only accounts for the pitch of the instrument. There are two other things to account for:
First, bass and guitar usually combine to create music across multiple octaves and when the two instruments are too close in pitch, things start to sound a bit muddy. So you want a wide spectrum of pitches as a rule (and variety in your tones and rhythms too). If you are trying to replace the bass with a single guitar, you're going to lose the ability to fill out the band. However you could use a second guitarist in to higher registers, with different tone, and rhythmic variations.
Second, tone is pretty dependent on string thickness and tension. When you add strings to a guitar, they're generally much thinner than bass so that you can still support a reasonable action on your higher strings. Further, when you tune down, you lose tension on the string. These two things combine for a flabby sound that has a hard time achieving punch. So even in scenarios where the guitar is in to bass territory, the bass is often used to add punch and attack to the sound.
So it's more complicated than just the octave you're in. However, people do all sorts of things against the grain successfully, so it's certainly possible, but there are good reasons why there's normally a bass player. Funnily enough, I came across an interesting video of a guitarist playing both the bass and guitar parts just this morning (albeit with a pedal rather than low strings and drop tuning):
yossarian has mentioned why a bass might be tonally more appropriate than a guitar; I'll add that the string spacing (as well as the thicker, tauter strings) make certain percussive techniques easier on a bass.
Remember too that the standard-tuned bass is seen by many bassists as not going low enough - 5-string basses starting at B0 are common, with extended range bassists going down to F# below that or even C# below that.
There's also the separate question of, regardless of the instrument, the benefits you can get from having an instrument dedicated to playing a monophonic bassline. Two disadvantages of having one of the guitars doing bass duties are 1) the guitarist is probably also busy playing other notes, making him less able to perform the tight phrasing and particularly muting that a good bassline needs, and 2) if the guitar is going through distortion, the distorted mix of strings is liable to lead to a mushy sound with less distinct bass attack.