I was thinking something along the lines that people will generally pick the very best things, ground projects, or whatever to do first, and so as they satisfy those they have to go on to not quite so awesome things, and so on. So although years per se don't 'get in each others way', how you spend them will.

If you are arguing that we should let people die and then replace them with new people due to the (strictly hypothetical) diminishing utility they get from longer lives, you should note that this argument could also be used to justify killing and replacing handicapped people. I doubt you intended that way, but that's how it works out.

To make it more explicit, in a utilitarian calculation there is no important difference between a person whose utility is 5 because they only experienced 5 utility worth of good things, and someone whose utility is 5 because they experienced 10 utility of good things and -5 utility worth of bad things. So a person with a handicap that makes their life difficult would likely rank about the same as a person who is a little bored because they've done the best things already.

You could try to elevate the handicapped person's utility to normal levels instead of killing them. But that would use a lot of resources. The most cost-effective way to generate utility would be to kill them and conceive a new able person to replace them.

And to make things clear, I'm not talking about aborting a fetus that might turn out handicapped, or using gene therapy to avoid having handicapped children. I'm talking about killing a handicapped person who is mentally developed enough to have desires, feelings, and future-directed preferences, and then using the resources that would have gone to support them to concieve a new, more able replacement.

This is obviously the wrong thing to do. Contemplating this has made me realize that "maximize total utility" is a limited rule that only works in "special cases" where the population is unchanging and entities do not differ vastly in their ability to convert resources into utility. Accurate population ethics likely requires some far more complex rules.

Morality should mean caring about people. If your ethics has you constantly hoping you can find a way to kill existing people and replace them with happier ones you've gone wrong somewhere. And yes, depriving someone of life-extension counts as killing them.