Take this as the result of a quick reading of the summary. My comment
as a non-expert is that probably a good, reliably working convert-ly
is a substitute for syntax stability, because it is precisely what
will make your documents compile on the long term. Your personal,
misfortunate case would not have taken place. And Mutopia would
compile in a higher percentage. I don't know if this is technically
doable or how many hours/man would it take.
So in any case I don't think we should get obsessed with syntax
stability if we can make convert-ly rules work better. Or if lilypond
were able to read old files and convert-ly them on the fly, issuing a
warning and suggesting the user to make changes permanent.
I'd like to ellaborate a bit more, later.
--
Francisco Vila. Badajoz (Spain)