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.