I disagree that there are too many people. Have you read much about the Malthus/George debates? Malthus has now been wrong for 200 years (and counting).
We can have population growth and still solve the impending environmental disaster.
On top of that, a declining human population results is an increasing young:old ratio. Fewer younger working age people supporting an increasingly older population. Socially and economically that will be a disaster, which will make solving ecological issues significantly harder.
We can have growth, but the growth rate must decline over time. For example the current growth rate of world population is around 1.1% per year. It is easy to see that it cannot remain above 1% for more than 12101 years (assuming FTL travel is not discovered).
Proof: Right now every living human is either on Earth or close to Earth. Without FTL it follows that 1 year from now every living human must be within 1 light year of Earth, 2 years from now every living human must be within 2 light years of Earth, and so on. 12101 years from now every living human must be within 12101 years of Earth.
If you calculate the volume of a sphere of radius 12101 light years centered on Earth and divide that volume by what the population would be after 12101 years of 1% annual growth you get 0.04 m^3 per person.
But the volume of an average human is ~0.06 m^3, which is greater than 0.04 m^3.
At 0.1% annual growth it would take a little over 127000 years to run out of room.
At 0.01% annual growth we've got 1.34 million years.
Of course the actual limits are much lower because the above is assuming that we can pack humans so that there is no space that is not occupied by humans which we cannot do because, among other things, (1) our shapes don't fit together perfectly to allow such tight packing, (2) there isn't enough mass in the 12101 light year sphere to make that many humans, and (3) we would need to use much of that space for the infrastructure needed to support humans.
Humor aside, it's a depressing worldview to me that each marginal human is a net negative. I take the opposite view, that each human is a net positive - and we need all the brainpower we can get to solve all of our problems.
We could solve a lot of our problems right now if people were prepared to change how they live and accept a decent amount more responsibility (e.g. sort waste by hand, etc.). Scaling up problems typically hasn't helped, and you get novel issues at large scale (e.g. parasites and diseases thrive due to monoculture in huge farms). As Bill Burr noted, we could all drive tanks to work if there weren't as many of us. It's cynical, but there's more room for largesse at small scale.
Pyramids were built with slave labor. Poor sanitation lead to the black plague. Imagine if they all stopped having kids, because they thought Egyptian slavery would never end and the black plague was insurmountable?