Being immortal and having a high quality of life for as long as we are alive are not mutually exclusive. In fact, don't we have longer lives and better quality of life than our ancestors already? This would just be taking it one step further. Also, having a population that never dies has many more benefits than consequences. The amount of knowledge and experience that will not be wasted because of death is extremely valuable. Also, who says people will be giving birth to children as often as they are currently doing when they don't fear dying. Already in most developed nations the rate of population growth is plateauing. Also, don't forget we have an entire universe at our fingertips. I'm sure when we have 200 year old aerospace engineers and physicists we will be able to get to colonize other planets much faster.

Regarding quality of our ancestors: Yes, but there are downsides too. We probably have longer periods of lower quality of life. We improved both quality and length. But we improved length more.

I hope for more youth, not just more life.

People are and always will be free to die whenever they want. If you live a longer youth and then face a longer low quality life, either you want that longer low quality life more than you let on, or you cut it short.

Without access to some humane means of euthanasia, I think your statement is inaccurate.

Living longer might naturally force people to take a more long-term outlook on the environment.

Also slowing down growth is much more palatable on a basic level if you have that much more time to enjoy what you have and get what you don't have yet.

I think that as people live longer they will have less children as a result, simply because the inner need to procreate and keep humanity alive will die down. We see this in the differences between the first world and third world countries where mortality rates have a significant delta. For instance, In Africa they tend to have more children on average than they do in Europe.