Although of course it seems unlikely, all good things must come to an end and someday, in the distant future, the blockchain technology will fail. When it happens, what could replace it? Is there anything out there that functions similarly and just as good and reliable as blockchain? More importantly, if nothing replaces it, what will happen to cryptocurrencies then? Any ideas?
Blockchain as a system justifies itself so far and will justify itself for a long time. There are no problems with it - it functions as clearly and correctly as possible and will develop and function further. If you mean what will happen to bitcoin if it is fully depreciated, then another cryptocurrency will come in its place and the process will be repeated again.