It is easy to agree that 'diversity' is valuable, but what kind of value is it exactly?
Take linguistic diversity, passionately defended by anthropologists and linguists who want to preserve all 6,500 thousand spoken languages from extinction. If you look carefully at their arguments, they are all about the usefulness of languages and not about the value of diversity per se. For example, it is claimed that minority languages contain useful local knowledge about plants, animals, ecosystems and so on which would be lost if those languages died out. Or that minority languages are important for individual self-esteem and community functioning among ethnic minorities, which in turn supports their flourishing and well-being.
However, if the value of diversity lies in what it does for us rather than what it is in itself, then whether and how far diversity is valuable (or even disvaluable) turns out to depend on the context. Diversity has no value in its own right; it is merely one tool among others that we may use to achieve things that do matter. For example, if we are worried that important botanical knowledge achieved over many generations may be lost as the last people to speak the language that encodes it die off, then it makes sense to send academic botanists to collect that knowledge and publish it in a universally accessible scientific database. But it makes little sense to try to preserve that knowledge by somehow getting more children to learn the language and then continue living in the same precarious relationship to the natural ecosystem in which that specific knowledge would be needed and communicated. That confuses the substrate with the information it contains. It would be like trying to preserve knowledge of Euclidean geometry by teaching everyone ancient Greek.