Worstgen
Shiroyru
Anime movies usually cover a single volume worth of story, and Gojo vs Sukuna was a volume long. However, fights are inherently faster paced in anime than non-fighting scenes are, so the movie would only be like an hour long if it just adapted nothing but the fight. If they did make it into a movie, they'd probably have the first 15-30 mins be Gojo interacting with his students one final time
Shiroyru
There's also the issue of how to end the movie. For anime-onlys, ending the movie at Gojo's death would not only be a downer ending, but also pretty abrupt. It'd also be kinda weird to end the movie with Kashimo just entering the battle. I think it might be best to actually include the Sukuna vs Kashimo fight in the movie too and end it with Yuji and Higuruma entering the scene
ZenZu
It would work, as long as they ended with Yuji entering like you said. My only question would be, would they just re run the movie in weekly segments for multiple weeks? Demon Slayer did that bs. I don't like that. They kinda have to for continuity purposes.
Set up everything in the anime and go into the movie for an hour of a hlaf of fighting.