This is an interesting conversation.
Sondheim is extremely generous and open about his shows; correct me if I’m wrong, but I actually don’t know of any show of his he hasn’t approved of. I personally strongly disliked two productions of his shows I saw in which actors simultaneously played instruments. To me it a) takes away from the story being told as you cannot suspend disbelief and b) is a cheap way of cutting costs by not having to pay an orchestra. Personally, I hope this is a passing phase!
But on the other hand, I 100% agree with @Soozievt that playing an instrument well is a big plus, and there are definitely cases in which you are cast in part because of this skill–straight shows included (even Shakespeare).
However, to those whose kids don’t play - like my D - don’t panic. I think the aspiring performer should diversify as much as possible given their own personal skillset. If that means branching into creating films or doing stand-up comedy or writing scripts or whatever, do so. The one thing that’s risky is to think that with your singing and acting and dancing, you will graduate and walk into an equity show and be more or less set.
Know your skills, know your type, build connections, market yourself.