@Scaramouche, I’m not sure what happens if a j-pouch gets polyps, but if it fails, yes, you have to go to an ileostomy. Sometimes they leave the j-pouch in place, but usually they have to remove it.
If you live somewhere with expert surgeons (e.g. USA), they can sometimes fix a faulty j-pouch depending on what’s wrong (called a revision) or even create a whole new one (called a redo).