Client source/code centralization is not terrible; it's a strength in many ways. On the flip side, if there were 25 (or 1000) different Bitcoin client implementations, that is now more attack surface due to the developer mind share being spread across those un-related clients. And likely resulting in dis-jointed and incompatible "features".

The Linux Kernel for example benefits greatly from the combined attention to its implementation, and all derivative distributions benefit. Optionally applying patches for their self use-cases; and when it makes sense supplying PRs back into the kernel. Much like Bitcoin Core and Bitcoin Knots, on a much smaller scale.

If there is a hostile or controversial change to Bitcoin Core; there is always the "Fork-it" route. See Bitcoin Cash/BSV, Lightcoin/Doge, etc.