The altar dragons actually respawn, it just takes some time. Perhaps do some fast traveling between Riften and Markarth, or use Wait (but not at the altar). Many reports are that re-spawning takes 30 in-game days, but I've had certain non-cleared dungeons take only 10.

The altar dragons actually respawn, it just takes some time. Perhaps do some fast traveling, or use Wait (but not at the altar). Many reports are that re-spawning takes 30 in-game days, but I've had locations take only 10.

I just tested Skyborn Altar. 10 days was sufficient for the dragon to respawn. I find it fastest to look at the date on the map screen at bottom right, add 10 for your target, and fast travel the longest distances to pass time. Markarth<->Riften is ~26 hours.

From my experience if you arrive too early you will have to wait all over again, better to load back before the arrival.