I've played lots of steam games off line it's pretty good that way as long as the developers support offline mode which Star-dock do. Anyway doesn't sound like they have any intention of producing a non-steam version at present.
Maybe start a petition, if there are a few 100,000 people for which a non-steam version is that important they wouldn't buy otherwise I'm sure star-dock would consider it, but there would be costs for them so it has to be worth their while and most people are fine with it being on steam.
Can't say I've ever had an issue re-downloading anything I've bought on steam as it stores that you own it in your account, unless of course your account is hacked or something I suppose. But that could be an issue with any software purchase and download system.
So out of interest is it just steam people don't like or are they after a physical media version?