In terms of potential good done and suffering averted, why is it better to work on maybe giving more people 70-80 years of good life rather than working on giving everyone 200-300 years of life and then working on improving quality later?
Because
continued life has the potential for future good where terminated life does not. The longer your lifespan, the greater the total potential good it can contain.
And I don't mean good life. I mean
life. Life expectancy in my country is just over 60 years. In much of Africa it's less than that. People in Canada, the UK and the US can expect to live for two decades longer than most of those on average.
According to the UN, it's less than 50 years in a lot of African countries, and less than 60 in another lot. Meanwhile, their estimates still peg Canada, the UK and the US as ~80.
Expecting to develop an aging treatment that will increase those 40-60 years to 200-300 and somehow be affordable for everyone in those countries in any reasonable timeframe is honestly in my opinion significantly more optimistic than hoping for the Second Coming in my lifetime.
I usually detest saying things like this for obvious reasons, but I really think that a lot of first-worlders don't realise how good they have it. To you guys, natural causes is the Next Big Problem. To me, it's... a desirable outcome. It's better than dying of AIDS, TB, cancer, black lung, malnutrition, homicide, drug overdose... The point is, there are a
hell of a lot of issues in the world. Immortality is going to do sod all for someone who'll never live long enough to appreciate it, and would never earn enough to buy it anyway.