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.