More likely it will be a larger burden on the environment. If there are no future generations to care about then why bother investing in a better future?
the earth regenerates itself, provided we stay out of the way. if anything there's more motivation to care when the degredation slows down and makes change feel tangible.
not necessarily connected to the number of people but the type - if they are all conservationists that would be a net positive, not counting OTT population numbers.
we're talking about aging in advanced economies - those are folks with heavy footprints. if the west could be content with africa's growth and allowing for more migration, this problem is licked.