This is down to Bethesda following a pattern. This pattern is five DLCs for every Skyrim game and then a sequel is launched. That is what we have seen so far when you take a look back at the past Skyrim game releases.
Up to now fans of Skyrim have enjoyed Dawnguard, Hearthfire and Dragonborn. If Bethesda sticks with the pattern, there should be two more DLCs coming out before a Skyrim 6 is out.