»Never touch a working system«: When my former download app stopped working with Mac OS 10.12., I had a problem. And I was glad to find EASY YOUTUBE VIDEO DOWNLOADER EXPRESS (PRO). It's working very fine even with Mac OS 10.12. and the mp3-downloads come with a much better quality than those downloaded with my old app (128 > 256kbps). I can highly recommend it.
P.S., 2021/05/07: YouTube caused a d/l-problem again, the developer is working on a fix.