Hi Don,
You cannot call this 'lock-in' because nobody has forced you to use lilypond and you have not paid anything for it, and there is no intent on the part of the lilypond project to capture you and your money. I am unable to see how the concept of deliberate commercial 'lock-in' applies to open source projects.
Lack of export formats in lilypond is not a fair criticism, especially since there is no universal music notation exchange standard, and MusicXML is severely lacking in many respects, and is not necessarily going to ever become a universal standard.