I disagree. It's just as hard to write something that has a beginning, middle, and end. Probably even more difficult than writing something that you don't have to write a final end.

A season of TV still does have to have an ending. A miniseries is just a definitive ending. A season of a running show has the luxury of not having to figure out how to end it right away.

The ending is what makes something impressive overall. Something like Dexter, that had great seasons, has been taken down a few notches because of its bad seasons and bad ending. Even Game of Thrones, which was generally considered still very good going into the final season botched the ending and has lessened its impact.

Something like Better Call Saul will have longevity because it ended well. It's great seasons will live on because it ended well.

A miniseries has to consistently hold that same quality for all episodes while also including the payoff of a satisfying ending.

Something like Severance could shit the bed with a season 2 if they don't have it planned out (I don't think it will, just using a recent example). Like Heroes did back in the day. And even a miniseries could start with a great premise and pilot but just fizzle by the end of the series. So when a miniseries holds it the whole way and ends well, it's impressive.

Endings are hard. Leaving something open and not having to explain everything is a lot easier.