As long as the quality is still there and no real changes are made, Always Sunny is going to keep airing until they feel it's time to give it the curtain call. That might come up quicker because of the age of Danny DeVito, who joined in its second season. Not to mention some of the other cast members doing their own films and appearances on television. Either way, they still have quite a bit of time before filming a new season whenever that comes. Even though they are signed on for a few more year, anything could happen.
Down the road years from now, Always Sunny will be held in high regard compared to some of the mainstream shows of today. It has the checklist marked with the all stuff that is needed to run for a long time and become a modern classic. May this show continue to get all the respect it deserves.