Another way to put this is that for most pop music, you could often degrade gracefully to three triads of major or minor quality.

But even for something discussed previously on HN-- the Beach Boys "God Only Knows"-- this doesn't work. Wilson really wants a half-diminished seventh chord at the high point of the melody. One could perhaps degrade to a diminished triad in the accompaniment, but he's literally singing the seventh of the chord so it's there regardless. Substituting a major or minor triad there is a qualitative change and sounds suspicious.

If you're making your own instrument you might want to be aware that there's a "suspicious" sound that some types of consistent chord substitutions can have on certain classes of instruments.

For example, a bagpiper can make severe simplifications to the harmony or even change melodic intervals to fit what they have available. Audiences generally accept this because the strictures of the instrument have made that a common practice. (Even if you're unaware of the strictures, you've probably gotten used to hearing the result of the common practice.)

However, if one consistently employs major/minor triad substitutions on the guitar people are eventually going to hear that as a lack of quality. As in the Beach Boys example, this will often happen during key points. The guitarist may get lucky if the singer happens to fill out the missing note of the chord-- e.g., guitarist plays a minor triad and the singer fills out the seventh at the beginning of the chorus of "Last Dance with Mary Jane"-- but eventually the audience will figure out that the guitarist is missing a vital skill.

Anyway, unless your instrument is so novel it doesn't have any associations with extant instruments, make sure you have some kind of "escape hatch" so that skilled performers can play the chords they need. :)