If we could make them non-expiring we gladly would.
No perfect solutions for direct-sale, it seems. Either we have a public download link for everything but you have to enter 6 (and growing) license keys whenever you do a new install, or the download links are private and time-limited. Though possibly there's a non-fastspring option out there for infinite download duration, I think Steven at Soldak set it up on BMTMicro that way, though I'm not sure he set it up that way for all customers as I think it costs him extra to do that - dunno.
Ultimately if no-DRM-at-all is the most important thing for you, then buy direct from us, but if ease-of-redownload is more important, steam (or one of the others) is more important. And FWIW the steam version has no DRM on it (so you can copy it to your other machines and run it without steam running), you just need to run steam to actually download it.
