Futwick (AKA Orientalist Twat),

One can make the key signature read however one wants, in so-called "modern music." Perhaps you have not seen this, but composers and other notators of music have done it.

The custom you're saying is wrong is something developed for classical "TONAL" music—a system largely concerned with harmony and harmonic relationships. (Not for Jazz. Not for Romanian fiddle tunes.) The key signatures are part of a larger (holistic) system of relationships. Such relationships take/took precedence, within that music, over incidental melodic movements. Cf. the equal tempered system, again highlighting the emphasis of harmonic relationships over the context-specific tuning of melodies. Cf. Highland bagpipe notation, which requires no key signatures because its key in relation to others is irrelevant.

From the standpoint of MELODY: no, one doesn't find a major sixth and major seventh to be ubiquitous for the melodic descent when in minor. Those are leading tones in the ascent. In this sense, "minor" is much like a melodic MODE.