I'd been holding off on getting BE until I knew there weren't going to be any more expansions (Civ VI's recent announcement confirmed that) and it went on a sub $30 sale (summer sale did that). I've played it quite a bit in the past couple days and I have to say I like it quite a lot. With the right mods I'm enjoying it far more than I ever enjoyed V, though it has so few and not many modders willing to work on it so I don't know how long that will last.
Overall I'd recommend it quite solidly at the ~$20 it's running during the sale.
This is actually an assumption, and I would say is probably incorrect. Again, I am not sure if you are aware of this, but BE and regular Civ are different dev teams, so one product coming out with a new version does not mean the other suddenly get discontinued. Second, I have personally tweeted at the developer asking for news on BE a few months back(After RT and patches and such), he said, and I quote "Just you wait". Take that as you will, but it means something is coming, i'm willing to bet a second expansion, because in an earlier Interview, which may or may not have been before rising Tide, the devs had been asked about support for BE and content. He said "A civ game isn't usually considered done until it has 2 expansions" (In both of these instances I am quote Will Miller, one of the lead developers)
I would say Rising Tide is probably worth it at a heavily discounted price, especially if you already have BE, and if you enjoy futuristic games. Will it blow you away? Probably not, but it has some entertainment value.
Should you expect more content? Yes, you should. How much? Not sure exactly, i'd guess 1 expansion and perhaps 1 or 2 dlc's.
Those are my best guesses have followed BE since it was announced.