Of course the answer to 1 is 'YES!'. We're at or near the apex of a very large pyramid of species that in concert form the ecosystem that supports us. Without the pyramid the apex has nowhere to go except for going extinct itself. We need those 'lower' species a lot more than they need us.
How much harm do you think would be done if a popular species of tree in the US went extinct over the course of a few decades? Do you think ecosystems would collapse? Do you think the economy would suffer?
Turns out, this actually happened. 100 years ago, the American Chestnut tree accounted for 25% of all trees in north america. There used to be over 4 billion of them, then 99.9999% were killed by a blight. Today, only a few thousand exist in isolated groves. Few in the US know about this. People go hiking in the Appalachians and think, "Ah, such pristine nature." It is telling that such a profound extinction scarcely registers on any national measurement of health, quality of life, or economic prosperity.
As I said, most of the species we need to survive are domesticated. And most of the others we need are so prevalent and resilient that they'd survive a nuclear apocalypse. Moreover, this problem corrects itself. If a wild species we use becomes scarce (or demand increases), people start growing it. This has happened with paper farms, aquaculture, and even truffles. When it comes to existential risks, lack of biodiversity is not worth worrying about.
The problem with extinction is that you are throwing away a working code base that took 3 billion years to write. Every organism that goes extinct before we sequence its genome is an absolute tragedy.
That's not true. We know enough about ecosystems to assess the importance of various organisms. For example, biologists say the 30-or-so malaria-bearing mosquito species can be eliminated without causing trouble.[1] Doing this would save at least 1,000,000 lives (mostly children) and tens of billions of dollars per year. Like the American Chestnut, these mosquitos are massive populations that serve important roles in nature. Yet killing them would not cause problems for their ecosystems, let alone humanity. Our dependence on nature is not so great, and our knowledge of nature is not so small. A single species so obscure as to be unknown to us simply cannot be crucial to humanity's survival.
With the story of the American Chestnut and the biologists' assessments of mosquitos, I hope I've presented some convincing evidence that lack of biodiversity is not an existential threat. Is it a loss? Assuming genomes aren't sequenced, yes. But the same is true for the destruction of old books. While regrettable, I don't lose much sleep over either.
I'm not sure if I'm making my point in a way that it is clear enough, so let me re-try.
Mosquitoes and chestnuts notwithstanding, the fact that there are species that we can miss does not automatically extend to 'we can miss all species except those that we can domesticate'. Ecosystems are complex, not very well understood and messing with them typically has disastrous and unforeseen consequences.
Because of this species going extinct should worry you because one day a species might go extinct that you directly depend on without knowing it. It's like playing Russian roulette, only you don't know how many holes there nor how many bullets.
So far nature has been able to patch the holes in an admirable way, life tends to re-inforce. But there is a critical point at which that re-inforcement no longer holds. Of course sticking your head in the ground and singing 'lalala' will not make the problem go away but it at least affords you the luxury of ignoring it. This is a very large problem and it is sad that clever people like you won't lose sleep over either a lack of bio diversity or the destruction of old books. Resilience is a good thing to have, not a liability. Our dependence on nature is as large as it possibly could be and our knowledge of nature is surprisingly limited, especially of the intricate interplay between species.
A single species so obscure as to be unknown to us could very well be crucial to our survival. Let's hope that such species become known before we find out they were crucial after all.
This is not a game of single datapoints invalidating theories, this is gambling on a scale that we can't even begin to afford until we've moved off-planet and have multiple, completely independent biospheres capable of supporting complex life forms. And once that's established we could try to reduce the biodiversity in one of them on purpose to see how far that envelope could be reduced.
