Those animals now not dying, might cause further ecological damage. Ecosystems tend to have a rather delicate balance and viruses play their part.
Thousands of insects and animals species appeared and disappeared before humans existed, countless of times, so I wouldn't give that much weight to existent equilibriums; what we can do is being aware of the changes, e.g. if we detect a sudden increase in spiders population we can check what is killing the frogs and act accordingly