In an extreme case, during a systemic banking crisis, holding risk-free central bank-issued CBDC could become vastly more attractive than 1/5  BIS central bankers' speeches  bank deposits. There could be a sector-wide run on bank deposits, magnifying the effects of the crisis.
Even in the absence of a crisis, readily convertible CBDC could crowd out bank deposits – putting the two-tier banking system at risk. Then, the efficient flow of credit to the economy would likely be impaired.