Both King Mickey and Riku have gotten out of that Realm, its not like its impossible!
Yeah, but they were lucky. look at Aqua she's been stuck there for 10 year, it might be less for her but still she's been there for a long time. But there's no doubt there are ways to get out, Ansem himself did it before. We still don't know if it's so easy to get in, they might do it the same way Mickey did it in KH1.