Why is that a bad thing?

Economic growth is often tied to population growth and the free movement of labour. Japan's economy isn't doing very well at the moment, and has basically stagnated for the past decade. This contributes to their low birth rate since people don't want to have kids when the economy sucks.

Less humans means less stress on natural resources, more space, less strain on the environment, few carbon emissions etc.

Places like Hong Kong and Singapore have thrived despite having few natural resources and a huge population inhabiting a very small space.

Also, it's pretty well known that as economies develop and people move into the middle-class wealth bracket, their birth rates drop. That's why models of human population growth show the population capping at around 10 billion. We aren't headed for a population overload.

And with automation coming fast, the "aging population" thing won't be much of an issue since less people will be required to work to keep the economy running.

Eh, the problem is more with the resources that are needed to support an aging population. Taxes will be raised in order to fund the growing cost of their pensions and healthcare.