Multiple implementations means more lines of code and hence more bugs. It will cause frequent chain splits and encourages a new range of sybil attacks.
This comes down to one's central belief as to what 'Bitcoin" is.....is Bitcoin a collection of consensus rules? Or is it a software maintained by a very small group of people?
If your argument is that Bitcoin is a software run by a small group of people (eg. those who decide what gets published every time there is an upgrade), then Bitcoin is much more centralized than most central banks.
This also comes down to another important question as to who exactly should be running a full node? It is my belief that businesses and those who are going to receive bitcoin prior to giving anything of value to their trading partners are those who should run a full node. Anyone who sends valuable property to someone prior to receiving bitcoin is already at risk of outright not receiving bitcoin in return, so the value of running a fully validating node is minimal. Further, it is the responsibility of those who are running a full node to personally ensure the software they are running is going to preform as they believe it will, and to not rely on third parties for this.
I agree both problems you are mentioning are very crucial but I afraid they are not relevant in this context.
Definitively bitcoin is a protocol and as a protocol it should be implemented somehow. One might buy a closed source or code down to the metal a proprietary software and participate in the protocol but when people use an open source implementation for this, things become a bit more complicated. In other cases it would be users own responsibility to take care of their safety but for open-source software it is community's credit that is being spent and ensures users about their security.
Obviously, it would be much safer for a community to take care of one implementation with fewer lines of codes.
As I understand, your concern is governance, but it is another topic (an unsolved one, I suppose) and there is no guarantee we could have an optimized solution for both immunity to software bugs and governance at the same time in one single package, at least right now.
As of your assertions about the limited value of running a full node, I do agree again, but when it comes to an implementation used by a significant number of users, this value accumulates to sensitive levels and people may lose both considerable amounts of money and (more importantly) their faith in the whole ecosystem.