Suppose there is an exchange that happens to be connected to some nodes that are vulnerable to some kind of attack. Or perhaps they aren't connected directly to those nodes, but connected to node which are connected to those nodes. Suppose this attack causes those nodes to go offline or otherwise become disconnected from the network. Even if there are a very small number of these vulnerable nodes, if they happen to form a ring around the exchange, an attacker can attack those nodes and cause the network to partition.
But you just described Sybil Attack which can happen with or without multiple implementations of bitcoin. It doesn't even need a vulnerability or hashrate to happen.
I do say we need more implementations of bitcoin
from scratch. It won't be a perfect solution and it will take time for the software to reach maturity of bitcoin core but the benefits of it are more.