if this is the case, it means that bitcoin core will no longer work for new clients (who have never started it), if the DNS is unavailable.
Is it a certain way of centralization? : /
No. Back up nodes are also hard-coded into the source code. In the event that the DNS fails, the client would then try to connect to the reliable nodes that are listed within the code itself. If all else fails, the client has to manually add the IPs himself using the command-line option which is not difficult.
The client would try to establish more nodes to connect to by the getaddr message sent to it's peers and build a peers.dat which includes some nodes which it has succesfully contacted previously.