Multiple implementations are necessary. If one implementation goes down or has a bug, then only the users of that implementation are affected. If there are many implementations then the effect of one implementation having a problem is minimal.

The fact is that there are already many implementations currently running. This site (https://bitnodes.21.co/nodes/) lists 83 different implementations that are currently in use. Unfortunately, they are mostly versions of Bitcoin Core, so they all tend to have the same flaws.

Also, there is an important flaw in the single implementation argument. Whenever the single implementation is updated, a new implementation that could create an accidental fork is published. You can't avoid accidental forks by using a single implementation if you want to upgrade it. In fact, there already has been an accidental fork caused by a Bitcoin Core update.