AT&T merged with Time Warner, which owns HBO. They want to give HBO/Go/Now and HBO MAX separate identities and keep HBO separate from MAX*, so you will still be able to sub to HBO Now by itself if you wish.
*HBO content will be available on HBO MAX though. Also, if they wanted to keep the HBO brand separate and distinct from HBO MAX then they should have given HBO MAX a different name like what the fuck lmao
So, just to clarify a few things:
Once Max launches, there will be a yet-to-be-announced plan to encourage current NOW users to migrate over to Max. You can continue to sub to HBO NOW, but the primary focus will be HBO Max once it launches. WM will continue to support HBO NOW, HBO Go, and MAX Go for the forseeable future due to existing provider and contractual agreements. I'm not privvy to plans regarding Go subscribers, as that's now a different org than the one I was in.