It can continue until people start withdrawing physical cash in large amounts, which will happen when the negative rate starts outweighing the cost and risk of storing cash.
After that, further decreases of the interest rate can only work if cash withdrawals are restricted to prevent bank runs, or alternatively if the value of physical cash and digital deposits are decoupled. The IMF has posted something about the latter option[1].
If interest is -50% those bonds must truly be worthless. Seriously, if you can't find solvent borrowers at -49% something is really wrong with your economy.
Considering that central banks for highly industrialised, wealthy nations normally move in 25bp increments, moving from -50bp to -50,000bp seems... well, a lot of central bank meetings. Are you proposing run away deflation? We have never seen it since the industrial revoluation stated. If the risk is real, why aren't we seeing more academic papers about this subject?
After that, further decreases of the interest rate can only work if cash withdrawals are restricted to prevent bank runs, or alternatively if the value of physical cash and digital deposits are decoupled. The IMF has posted something about the latter option[1].
[1] https://blogs.imf.org/2019/02/05/cashing-in-how-to-make-nega...