In your scenario, the TOTAL years lived remains the same in both cases, but the AVERAGE utility goes far down in the second case. 80 years per person is a lot less than 800 years per person.

Not only that but there is a decent claim to be made to -- within certain bounds -- noting that ten people who live only 100 years is less preferable to a utilitarian than 1 person who lives 1,000 years, so long as we accept the notion that deaths cause others to experience negative utility. The same number of years are lived but even without attempting to average utility the 10x100 scenario has 9 additional negative-utility events the 1x1,000 does not.