We may not need central banks, but the government and the state as a whole need central banks. They perform certain specific functions in the state and as an organ of the state, it is an integral part of it. Therefore, I think that central banks will always exist as long as states exist.
If we need government, then we need it's parts also.
But it is not necessary for government to have a central bank, it needs it just in modern economy situation. If economy would change, if it would become more effective and decentralized, oneday it could be so that there would be no need in banks while the governments would still exist. What do central banks do nowadays? Regulating inflation, economy growth etc? But if the industrial produce would become automated and robotized, oneday, I hope, people would not need to go for work to get money for their base needs, and then there would be no need in inflation or regulation of economy growth. Maybe oneday there would be no banks at all. Maybe...