When interest rates are negative, there is no other option than to fund current obligations from equity. You can't do it from interest, because there isn't any.
The argument against negative interest rates was that people could simply store cash in a vault and get a 0% return, rather than lend it for a negative return. In practice very slightly negative rates seem to work out because storing large amounts of cash in a vault is hard. However, it's probably still right to assume that if interest rates are meaningfully negative for a while everyone will switch to storing cash.
Help me explain where my unfounded assumption is. If the interest rate is negative, that explicitly means that money in the future is worth more than money today. That's the definition of a negative interest rate.
Much depends on the exact phrasing. Offering negative interest rates on consumer accounts is ludicrous to the point of rendering it impossible. For a national bank, nah, not so much.
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