Schubert was not the first composer to use the term to indicate what Alfred Einstein called the "lyrical quality." Clementi used it from time to time; Haydn used it often. Now, revisiting Haydn intensively, I am struck by how many problems are solved by interpreting the word in this way.
Like Schubert, Haydn was very much a vocal composer, though most of the time he is not characterized in this way. But he heard vocal inflection in every instrumental piece he ever wrote, much as Bach did. This was not only the highest compliment that could be paid an 18th-century composer, it reflected the soundworld they knew and which was all around them.
When I worked as a chamber musician, both performing and teaching, I was struck repeatedly by the lack of vocal awareness on the part of most players, both amateur and professional. Whereas amateurs were curious about this, however, professionals had a way of turning up their noses at the notion that text might underlie some of their precious sonatas. Does this signal a simple-minded notion of what words convey?