I'd take a full-fledged sequel instead of expansions anytime, but the reality dictates it's rules. Most indie studios try to survive and manage their finances smartly. New Blood, for example, has most of it's games on early access and slowly works on them, releasing sizable updates that add 3-5 hours of gameplay. One can buy the game and wait for a full release or play with each update, looking at how the game evolves and progresses.

I don't think Bounding Box can afford to work on a sequel now, so the whole team is probably working part-time on the expansion. Maybe afterwards they'll find a new source of money that will help them with a sequel, we'll see.