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.