Theoretically, Dish could just serve the content directly over the net, but can't right now due to licensing issues.
From a legal perspective, Sling gets around (or tries to) some of the licensing/copyright issues since you are playing back you're personal DVR for your personal use. Some of the networks disagree with this and have made Sling a bullet point in the PTA/Autohop suits. Time will tell.
Technically, the root of what the adapter does is to re-encode/re-compress the image to fit your bandwidth. Without Sling, you would need about an 8mbit upload rate to play back a Dish HD sat broadcast natively. We are beginning to see those kind of upload rates, but they are a long, long, long way from being the norm.
Bottom line, if we want the functionality we will need a sling adapter (or sling box) for the foreseeable future until licensing, copyright and bandwidth issues are resolved.
Will it eventually be obsolete? Yes, but so will broadcast/linear TV in general. I'm not sure I'd take a bet on which goes first.