Why? You're absolutely right. As is
@ixlramp above: that bass cabs can't reproduce the fundamental of low F# is a moot point, because the vast majority of them also can't produce the fundamental of low B to any appreciable level, nor can many of them do a fundamental of low E that is louder than -10dB from the upper bass. However, I see no-one arguing against low E tuning and suggest we all use baritone guitars instead 'cuz any lower than that would be inaudible anyway through standard equipment. Because in fact it is plenty audible: if low E (as in, the tuning and its lowest note, not the lowest partial of the latter) works through average quality cabs and combos, and low B (tuning and note) also can through non-crappy gear, then the truth is the 2nd and 3rd harmonics are what counts, and the brain fulls in the missing info. (
Vd. the so-called "missing fundamental" effect; or, as someone put it, why we can tell if it's daddy and not mommy at the telephone or doorphone even if their bandwidth is super narrow and devoid of any bass.)
No and here's why. What happens when one tunes lower than standard is that not only the elusive, oftentimes virtual fundamental goes down, but, also, the density of harmonic partials per a given octave increases: within the slice of middle/upper bass that the cabinet or combo does do well, we hear, say, four harmonics instead of two (their intensity relative to one another will vary according to the tone of the system - string, pickup position and inherent response first and foremost), and that passband of the cab response usually has a bump of a couple dB.
Now: do you, the player, like hearing those bunched up harmonics when you play a 27.5Hz A-note instead of our run-of-the-mill 55Hz one? If you do, keep at it by all means; if you don't, well, either tuning down to low F# is not for you or you do need to consider finding a more capable speaker that is able to eke out an additional, audible lower harmonic that reins in the aforementioned bunch.