I am personally a supporter of alternative implementation of Bitcoin because of this reason:
If the first version screwed up (eg. value overflow in 2010 and in 2018), the user experience would reflect badly on Bitcoin.

With multiple implementations of the protocol, the chain's survival is ensured because for example in 2010 when the chain started growing with invalid blocks the alternative implementation would have rejected those blocks and any miner running that full node or multiple nodes would have built the correct chain ensuring that bitcoin network doesn't halt and could even prevent fraud such as double spends when the invalid blocks were removed from the chain.