KSP as it first hit Steam was fun, but obviously incomplete. KSP 1.0 was viable as a finished product, but had obvious, planned upgrades unfinished. After-release support was pretty obviously planned from the beginning.
As for how long, though, I don't know. I'm exceedingly happy with the KSP I got for my money, and unlike some games they could have quit support before now without upsetting me. So as long as Take-Two keeps a decent business model, I'll happily pay for further content.