Soarin' isn't really the signature at all at DCA. Grizzly River Rapids (why cant they have this in AK?), California Screamin', ToT all top it. Soarin' is not really popular at DCA and the previous poster is correct in saying its a walk on most of the time. There can be 30 minute lines at Tot and walk on at Soarin'. I have always been baffled why its so popular at WDW. Even when it was brand new at DCA, this attraction never drew in comparison with the other rides in the park. DCA is slower than Epcot, but even so other things would line up a lot more.
I still doubt they'd close it, although they are doing massive retheming and construction at DCA. Even then, I know they are doing each part section by section, first being the pier, last being the entrance. The area where soarin' is will be themed to match a new natural theme that grizzly river is going to receive that will match the existing playground theming (rumored animatronic dear, bears, etc., yay!). I think thats a year or two away and even then I would presume they'd fit it in because the ride can put through big numbers even if its not so popular as in WDW.