Google negative yielding sovereign debts market cap.
USD 18 TRILLION https://www.bloomberg.com/news/articles/2020-12-11/world-s-negative-yield-debt-pile-at-18-trillion-for-first-time
I don't think anyone has ever said "only". But can you give even a single good reason to purchase a negative-yield sovereign bond?
The only reason I can think of is a bad reason -- that your banking license will be yanked if your bank doesn't act like a good little chartered institution and enable the government's debt auctions.