As not all dementias are the same (there are several types) the Phase 3 study would probably be in the same type of diabetic patients to confirm the subgroup data.
Concurrently BP might want to do a second trial in patients at high risk for other reasons (family history, apo E4 genotype...) to broaden the market.
I think a prevention trial would be too latge and too lengthy at this stage.
All imo