That's what I and other thought when Diablo III was announced to do this: most of their games have been online centric and they even have WoW, so it must be rock solid, right? That was an erroneous assumption, and many of us didn't like it out of principle anyway. I expect similar can happen here, and even just staying at the same level means there's a good chance I'll be annoyed at random outages if they go all in and don't budge.
They've been building XBL for this for a long time. It is the backbone of everything they have been doing for a decade. If these rumors are true (sure sounds like they are to at least some significant degree), they aren't going to half ass it or be surprised at the load on their servers. XBL doesn't go down when the new COD launches, does it? I'm not defending always online, as I don't want it, don't like it, and I don't see the need for it, but I can't find it in myself to freak out over it.
Really, I want to see what advantages there may be. For example, if it means we get a break on the console price or content price, maybe it's worth it.