Our existing lifespans are arbitrary. They are largely based on the length of time needed to procreate and ensure a successful subsequent generation or two; and on our ability to overcome that limitation. Saying, "I would not take immortality treatments if they were available" is different from "I will not accept medical help for my heart condition" only by a matter of scale. Having more humans on the planet and for longer does come with challenges, including resource usage issues, but it also comes with the tools to conquer those challenges and other benefits besides.
The cost of mortality is having Albert Einstein around for a meager 76 years, instead of hundreds or thousands. Multiplied by all the other great minds our species has produced, this is an incredible cost that, in my opinion, vastly outweighs your assumptions about the cost of immortality.
In my opinion, culture can only be threatened by homogeneity, but even print, radio, television, film, and internet haven't managed to kill it. Immortality will necessitate and possibly facilitate cultural change. First, because our culture will have to adapt to accepting immortality. Second, because immortal beings will likely become bored. We really have no indication that culture will stagnate or die. I suspect it will slow, but I suspect most things will slow when we are no longer so bound by time. And, due to resource issues, immortality will likely require (and possibly result in) a great diaspora of humanity. More humans, living in more places, having more experiences, and being more separated, means a more diverse culture, even if a lot of those humans are the same humans.
Finally, immortal humans will likely be far better caretakers of resources. They will have personal incentives to make sure there are enough resources, that those resources are renewed, and that they are used in a way that preserves the planet. Humans have a very difficult time even conceptualizing what the world may be like for their great-great-grandchildren (and modifying their behavior accordingly), but they understand fear for their personal safety far better.