As a semi-professional trombonist, C clefs are very important to me. Alto and tenor as-written, tenor as a means of transposing Bb treble, mezzo-soprano as a means of transposing F parts… they are not obsolete.
In particular, trombones have almost entirely stopped using the tenor clef in jazz.
Commercial players are likely to use the skill for transposing Bb treble lead sheets, or playing in a pit. Tenor clef shows up in modern musicals regularly.
Not to mention being shunned from quartet or choir opportunities that are the basis of networking in the industry.
And even trombonists have to learn how to handle sharps and flats in chromatic passages, so there's a lot more to transposing than using a different clef.
> Tenor clef shows up in modern musicals regularly.
This I didn't know, thanks!