On monoculture and diversity...
There absolutely should be one coin/ledger but is it imperative that there be one protocol/chain to update it?
Maybe multiple chains could allow us to manage the risk of one failed blockchain bringing its coin down with it?
And what if the coin itself fails, perhaps for economic rather than technical reasons?
The term "coin" is introducing confusion. The question is,
what if the ledger fails? This makes it clear that we must be careful to distinguish between the ledger (a conceptual spreadsheet keeping track of who owns what percentage of the economic community known as "the Bitcoin ecosystem" and later perhaps just "the global economy") and the protocol for updating that ledger.
Now here I wondered how you could possibly mean that a ledger - a mere spreadsheet - could "fail" for economic reasons. Then I noticed later you posted
this, which is a common economic misconception that I recommend rooting out of your thinking completely:
Also your example of physical gold is not really an argument in favor of bitcoin because gold does not have a fixed supply and also has a supply that is responsive to technological advancement (correlated with economic growth).
The problem is in imagining that the money supply needs to expand to accommodate an expanding economy (or contract in an contracting economy). As a ledger, all that matters is what percentage of total supply you own. Disregarding the physical difficulty of working with very tiny pieces of gold, even a single ounce of gold would be enough to power the world economy, no matter how it may grow or shrink. Any amount of money works the same, because again it just comes down to what percentage of the total supply you hold. The term "1 BTC" just means 1/21M of the total ledger (or 1/13M of the current ledger). 130,000 BTC is just another way of saying "1% of the current ledger." An ounce of gold is just another way of saying X% of the total gold supply.
Laboring under that misconception, and thinking in terms of "coins" instead of a ledger (and these two confusions go hand in hand), I can see how you might think the Bitcoin ledger could fail due to there "not being enough money."
With that possibility out of the way, you are still quite correct that Bitcoin
the protocol could fail for technical reasons, so arguably a monoculture in protocols is bad. However, monoculture in the sense of everyone using the same ledger is not a bad thing at all, and it's kind of the point of money in the first place; the only way it could be bad is if the system for updating that ledger were faulty, which again points to the protocol as a possible weak point, not the ledger itself.
TL;DR: Monoculture in protocols may introduce weaknesses; monoculture in a having a single ledger doesn't, and is pretty much the point of having money at all.