It was setup in the end credits of the movie, not even within the main narrative proper which told a complete story. Isn't continuity a bonus?
You're forgetting the parts in the actual movie where Kang talked about the other versions of himself who exiled him, and warned Scott that he'd be unleashing an even greater threat if he defeated Kang. The post-credit scene just added to what had already been set up in the movie itself -- and set up even more explicitly by both seasons of
Loki. Indeed, it was the fundamental root cause of everything that happened throughout both seasons of
Loki, in the same way that it was the fundamental root cause of everything Kang did in
Quantumania. It's utterly absurd to suggest that this is something limited
exclusively to a single post-credit scene. That merely showed what we'd already been extensively told about.
Does it have to payoff in an Ant-Man movie?
Why are you asking me?
We already know that the plan is to pay it off in
Avengers 5, which up until recently was subtitled
The Kang Dynasty. This is not a secret. Presumably the plan is for other movies and shows, including ones not necessarily announced yet, to contribute their own setups to the building arc in the same way that was done with the various Infinity Stones.
That plan is, of course, now likely to change in response to real-life factors. But what we're discussing here is that there are good ways and bad ways to make that change, and the best way is going to have to be something careful that takes into account everything that's been set in motion and would be affected by the change, including the parts that we in the viewing audience don't know about yet. You can't turn a moving supertanker on a dime. You have to plan way, way ahead to account for its inertia.
Is it OK that it gets paid off in an MCU movie four movies down the road? Can we expect Joe Viewer to watch four movies for a payoff to an end credits scene instead of being in a TV show?
Again, are you forgetting that Thanos and the Infinity Stones were teased for
six years before
Infinity War?