I know this is probably the last argument most people want to hear, but is this not a case where more independent implementations would result in less risk? If you maintain that one particular client should form the "backbone of the network", you have to consider what happens if that backbone breaks. If there were a wider variety of clients being run, there may have been less of a threat to the network overall?
Core have done exceptional work, but at the end of the day, they're still only human. Assigning more people to keep an eye on one codebase might help mitigate faults, but if there's only one main codebase, there's still going to be an issue if an error slips through the net. Hence my belief that more codebases would create a stronger safeguard.
What many people do not realize is that having people run different implementations makes it easier for attackers to partition the network and thus harder to resolve situations where vulnerabilities are exploited. Network partitioning can cause multiple blockchain forks which is a much harder situation to resolve than a single fork or an entire network shutdown. It is not just that some nodes will go down and the rest are up and the network is still running. If the attack is directed in a certain way, miners will be separated and no longer connected to each other which then causes forks. Network partitioning is a serious issue, and running different implementations makes it easier for attackers to partition the network. So having multiple implementations and recommending that people run alternative software is really not a good thing.
That being said, having multiple implementations is good for the individual who runs multiple nodes with different implementations. With multiple nodes each with different software, attacks exploiting critical bugs lets them know if an attack is going on. If everyone ran multiple nodes with different implementations, then multiple implementations are fine. The network would not shutdown and there wouldn't be any network partitioning. But not everyone is going to do that.