Reading your example, it seems to me that the "FAIL" is caused by the hypothetical user misunderstanding the semantics of relative mode. Using relative mode without explicitly defining what pitch the phrase is meant to be relative to is unreadable and prone to break. If it were written properly, it would be:
\relative {
\resetRelativeOctave c'
c4 d e f g1 % a rising diatonic scale, starting on middle C
% if the next line is meant to start on the G above middle C, it should have a new \resetRelativeOctave
g'4 f e d c1 % written relatively, this means we care about the melodic leap upward from the previous note
% now I want to reuse the rising diatonic scale… so I cut and paste the first two lines:
\resetRelativeOctave c'
c4 d e f g1 % a rising diatonic scale, starting on middle C — no problem!
}
Relative mode makes perfect sense if you're entering music that cares mainly about the relationship between notes within a phrase (i.e. most music). IMO absolute mode might be easier from the perspective of the software, but it's not how most musicians think, and that's important. Maybe the documentation could do a better job explaining the semantics of relative mode and when to use \resetRelativeOctave?
I take exception to the idea that relative mode ought to be deprecated. I've been using exclusively relative mode to compose for almost ten years, and I think it's great.