I think the introduction of the word "definitive" is clouding matters here because, unless the director dies immediately after completing it, there's always an implied "(as of now)" after the word.
Trying to generalize isn't useful. Too many factors vary, not least of which is the personality and professional status of the director himself.
A director may publicly stand behind a theatrical release -- even if it was butchered by the studio -- because he feels that he has to in order to protect his future in the industry. Or even because he's the kind of person who can convince himself that the changes were for the better -- or has to convince himself just in order to keep working on the thing until it's releasable.
But when the opportunity presents itself to revisit the film, he becomes equally enthusiastic about his "original vision." These are human beings, after all.
Studio interference is certainly the most common reason that the theatrical cut is unsatisfactory. And very often the opportunity to recut a film without studio constraint produces a better film.
But the label "definitive" isn't useful, except as a critical descriptor. Sure, it's up to the director to say which version he thinks is most satisfactory to him, and call it a "director's cut" if he wants to -- but it's up to US to decide if it's "definitive."
So if someone wants to say that the theatrical cut is always "definitive," they can. It doesn't change anything. Perhaps it means this is the sort of person who is uncomfortable with open-endedness and incompleteness, who wants to know that once they've walked out of the theater they've actually seen the movie they just sat through, not just some transitional work-in-progress. Perfectly valid position to take.
I tend to feel the same way -- because almost always the theatrical cut was the product everyone involved was actually trying to produce -- but I look at each film individually, too. I would never be interested in seeing the theatrical cuts of "The Abyss" or "Kingdom of Heaven" again, simply because having seen the director's cuts it's evident how compromised the theatrical versions were. Yet I enjoyed both films (particularly "The Abyss") when I first saw them, anyway. The theatrical cuts were "definitive" until I saw the director's cuts, which then became "definitive."
LOTM is different. In the first DC, Mann made changes that feel like purposeless tinkering, serving goals only he can perceive. In the process, he he reshaped the section of the movie that "defined" the experience for me, changing it to such an extent that at the end, the experience was gone. The opposite of "definitive."
So we'll see what this new version brings. And then we can decide if it's definitive. I reckon that, for people who will have only seen the new cut, it will be. Because as with any art, it's defined by the beholder.