Comment by opo

>...Is the cross-elasticity of demand here not referring specifically to the cross-elasticity of different forms of labor, which exists due to the many common inputs of different forms of labor?

This isn't necessary to explain Baumol cost disease. Different kinds of workers may also be substitutes for one another, or complements. However, the quartet doesn't need to share a particular set of inputs with car manufacturing for Baumol's mechanism to operate.

>...In the case of a quartet, if musicians charged what they did in 1700, they would quickly become homeless, and then quickly get other jobs.

Yes. Baumol cost disease is a way to explain the upward pressure on the cost of lower productivity labor-intensive services. It isn't an economic law, it is a tendency. It doesn't guarantee that every business will be able to pass those costs on to customers, or that every business will survive or that every worker will receive the same wage increase, etc.