Now, I'm aware that nothing prevents anyone from attempting an alternative implementation and that someone could conceivably read the source code and proceed from there.
Alternative implementations do exist though. Like
BTCD.
Shouldn't the reference implementation be one of many?
There should be only one
reference implementation but other alternative options for people to run as full node.
..
BTCD is not a unique node that allows people an alternative path to make protocol Bips or upgrades features from.
read their comment on the link YOU provided
Debatable. Because the BTCD developers are outside the political process of the Core developers. It can also be debated that IF BTCD starts gaining a larger following, it can drop BIPS and create their own political process and compete with the Core developers.
In that hypothetical situation, it's probably A "good" for decentralization, but it's also debatable IF the "other implementation/reimplementation" is "still" the Bitcoin protocol.