I routinely find ways to play downtuned songs on my standard tuned 6 string. For example, for a 7th string rooted C5 chord, I would fret the thrd fret on the low E and A strings (G and C). Most people would play the C5 with an A string root. By putting that G on the bottom of the chord, it gives it more chunk. Your ear tends to hear the low C note that isn't actually there. Same like with a 7th string rooted D5. I hit the open A and D strings. You hit those good by themselves, with even a half-assed amount of distortion on there, it'll likely sound like that low D is there. An "aural illusion" of sorts.
For me, yeah, it may be cool to have that 7th string, but I'd find myself rarely using it. I like to play a 6-string. I deal with downtuned songs easily enough on a 6-string, with (honestly) minimal effort.