Pixel is a line of hardware products, and to stay competitive in Google's target market, new hardware releases are essentially necessary. Likewise the common thinking in the industry (correct or not, and most likely case-by-case) is that keeping production of older models up and running isn't viable. That's the same reason you can't get first-gen iPhones anymore.

With software, you have the benefit of not having to scrap the entire product at once -- you can keep the sites, domains, brand, whatever -- and because the vast majority of users on something like Hangouts used the official client, you could get away with arbitrary API and interface changes without losing (nearly as much) brand momentum.