https://en.m.wikipedia.org/wiki/Ecological_footprint
I think you are suggesting that earth can carry many more people than 10B. I’d like to understand how you think that is possible.
Furthermore, even at the current population size human activity is causing potentially irreversible climate change. The scientific consensus is that even a small increase of 1.5C will have severe consequences on the planet. Limiting the increase to 1.5 C will require significant action by all of us. Addding many more people makes this much harder.
http://report.ipcc.ch/sr15/pdf/sr15_spm_final.pdf
I hope the links can help convince you that we are facing a serious crisis that is causally related to the size of the human population.