I don't see where Adobe is offering Lightroom on a subscription basis, unless it is included in the full multi-package Creative Cloud. At $50 a month, that would be an expensive way to get an $80 software upgrade!
I think Adobe could make a lot of photographers very happy (and quiet down all the noise about CC) if they offered the Photoshop, single application, with Lightroom at (or close to) the single application subscription price of $20/month. I would certainly take that offer.