In decentralized systems, there is no security on top of network but if something big happens like millions stolen there can be a consensus to solve it and that means that all miners (in PoW) or witnesses (in PoS) agree to make a change. If there is no consensus chains split into two chains. We saw that with ethereum so now we have also ethereum classic.
But why is that? It is because no one controls coins except users. For some people that is a good thing, but for the majority of small people who are in the mass market waiting to adopt crypto, this is bad thing cause when hackers start stealing govs will jump in and make a mess.
Centralized systems like Onecoin solve this really huge problem that no one seems to realize only because they look too short at the current tiny market.