I think each of them and also 7 were interesting on release for some time for the novelty aspect at least. It‘s fine to be bored by any of them after 100 hours. I don‘t remember for the older ones, but it surely was the case for 5 and 6. I returned for single games when a big patch or DLC hit. And I had fun then. But otherwise the games were outplayed until larger changes happened.Are you asking if it's more boring than VANILLA civ 5 or 6? Or CivIII?
I‘m not quite at that point with 7 - I can still play it without new content arriving (but this is still extra motivation to start a new game).
Yet, the comparison with vanilla civs is a bit pointless. When I decide what to play today, a vanilla civ version isn‘t a possibility. So, there‘s not really a competition for boredom between civ 7 „vanilla“ and 6 „vanilla,“ but a competition between the current (partly modded) versions that’s important for me. And civ 7 wins for the time being
