I think your point is somewhat fair, and I agree that advancing medicine is not a great sole reason to keep around our fellow organics. There are a few ways to respond to this, including such notions as having intrinsic respect for nature, and appreciation for what natural selection has wrought. But for one and most simply with regard to your anthropocentric viewpoint, the ecosystems that these species constitute are actually critical to keeping civilization going in a practical economic sense.

The theoretical human civilization that can withstand a global ecosystem collapse and exist on a paved over earth is perhaps possible with the right technology but also it is 1. very dreary and 2. much more expensive and difficult to maintain than just putting in some effort to prevent ecosystem collapse now.

Fisheries management is a good microcosm for the cost of ecosystem collapse. If we manage a fish population correctly, we can continue to harvest fish from it and get resources out of it indefinitely. If we do not manage it well and let the population go extinct, we lose that pool of resources permanently and need to replace it with another equivalent source of food which may be very expensive in comparison. Perhaps the local human civilization which relied on the fish will be unable to adjust and will also fall.