What will I lose if I let iTunes Match expire, and instead subscribe to Apple Music? EDIT:  I just discovered that an Apple Music Family Plan actually requires OSX Yosemite and iOS8, which is a non-starter since we will not be upgrading past Mavericks (Yosemite caused me massive problems and I had to remove it and downgrade immediately) and my partner despises iOS later than iOS 6 and won't use it.  So I guess this means I'm stuck with only an individual plan regardless.

EDIT 01.31.2016:  Turns out that there is a VERY significant, but somewhat subtle, reality in dropping iTunes Match and using Apple Music instead.  ALL music you add in the future, will become DRM-encoded if you ever download it from the iTunes Music Library, including your own non-DRM CD rips!  Looks like the only actual "solution" is to have both iTunes Match and Apple Music.  =(

I've been an iTunes Match subscriber for a couple of years, and I have my ~15,000 songs all uploaded and also local on my Mac.  I don't really use it for mobile devices much at all, and primarily got it so that I could update my library (much of which was ripped from CDs long ago at 96kb to 128kb) to 256kb versions.

Now I've been playing with Apple Music during the initial free trial period and I have to admit, I like it a lot.  I've been discovering music like crazy including many albums by artists I like a lot, that I had no idea were even still releasing albums (OMD anyone??  WOW!)

But now, iTunes Match wants to renew again in a few days, and I'm really unclear on if I need it or not.  Info online seems to be incomplete and conjectural, since Apple Music is still so new.

What exactly will I lose if I let iTunes Match expire, given that I already have all of my library downloaded locally?

I understand (correct me if I'm wrong please) that with Apple Music, all matched music that is downloaded will have DRM.  I can live with this.  I also believe that any music that is not matched and instead is uploaded by Apple Music to my iCloud Library, will stay in the original format i.e. DRM-free.  This is good because some of my library is music I composed myself or by other musicians that I know or work with, and I wouldn't want that stuff to suddenly be locked down.

Given this, is there actually any reason I'll regret letting iTunes Match lapse and instead signing up for an Apple Music subscription?  I know the cost is significantly higher, but I'm so enjoying the music discovery process in iTunes that I'm actually thinking it's worth $15 for a family plan for myself and my partner.
Your understanding of the DRM is correct.

The functionality of iTunes Match is included with Apple Music, so yeah: win win.