I’m on the fence on this. Let’s accept that it’s possible to do this. A few of the implications of lifespans reaching 300 to 400 years are:
1. For humans, destined to live 400 years, the simple act of crossing the street would be dangerous. Many people would avoid all that and much of every kind of activity. This would greatly reduce the quality of life in all cultures and leave countries open to invasion and plunder(sort of already happening).
2. Jobs and careers would be held by Elders for centuries. Newborns would would have huge problems at age 20 of finding any worthwhile work. (Also sort of happening now ).
—— good grief—— I guess we are there now!
3. Overpopulation would require colonizing new planets to acquire additional resources and space.
Asimov wrote some science fiction regarding the issues that would arise with increased life-spans. 5 novels called the Robot series. (Robot Dawn, Caves of Steel, etc). A great summer read.
Living 400, or 4000 years for that matter wouldn’t change my risk perception any. I’d just expect death to come during something I liked doing.
Being a geologist there’s a crapload of stuff I’d like to see/record that ain’t happening with 100 years but hundreds or thousands would make possible.
Also, in general, people would be in a lot less of a hurry if the time frame was stretched out. If you had a several thousand year lifespan when it came time to take a trip to the other side of the country walking with a pack or a cart is conceivable. People wouldn’t be in a frenzy to try to manage earning a living during their brief existence and fill their “bucket list” at the same time.
You could actually get a plot of land and undertake projects with decadal time spans (I’ll build a cabin here when these trees are mature in 50 years, these beavers will have this rocky wash modified into grazeable pasture in 70).
If we lived for centuries the impetus to jam all of the feelings and experiences we can into the brief window we currently have our physical and mental peak in would vanish (given a few decades to sink in). Most critically, we could uncouple ourselves from the shackles the “elite” have upon us by preying upon people’s general fear of death and desires to fulfill all of their experiential desires in their brief prime health window.
You and I are - almost (see above stricken text) - on the same page here!
Those are exactly the same thoughts that occurred to me when I first began seriously contemplating this subject, 50-odd years ago.
Indeed, I, too, later drew parallels to those same Asimov novels. Asimov presciently describes the very social issues which would arise due to extreme longevity (though he attributes them instead to the effect of robots). In fact, I suggest that you now re-read those novels in this light, taking care to recognize the little clues and hints (and possible solutions!) which Asimov drops.
An extremely long-lived population would, indeed, be very risk-averse. Paradoxically, in spite of advanced medical technology keeping them at the peak of physical and mental fitness for an indefinite period, most individuals would be plagued by a perceived need for constant vigilence - always on the look-out for lethal hazards. Even only walking across the street could be viewed as an unnecessary risk (that's exactly the example I always cite). Of course, some people would adopt high-status thrill-seeking lifestyles.
On the other hand, due to the fantastic level of economic prosperity to be expected in the future, child-bearing would continue declining. Replacement levels would not be maintained - there would thus be no danger of overpopulation (on the contrary!).
Children (i.e., people under the age of 90 - who would not, however, actually be visibly distinguishable from oldsters) would then make up only a small percentage of the population. As a rarity, they would be viewed as a precious good, and have high status.
Due to the fantastic prosperity and technical advances, people would become very isolated. They would, also, increasingly turn to technology for the fulfillment of psychological needs (companionship). They would increasingly become unwilling and unable to form "normal" human attachments (resulting in a further decline in fertility). Catchword: "Bowling Alone."
To summarize:
A. Global population would actually decline.
B. Unemployment would be a non-issue, due to the immeasurable wealth of society. (Though you are correct in pointing out that many people would be unable to find gainful employment - though that expression will then be about as meaningful as "The divine right of kings" is today.)
All of that should sound quite familiar to us. All of these trends are visible today (as compared with, say, 150 years ago, when children were considered nuisances, to be seen and not heard; when it was expected that many would die from childhood diseases / not all would attain majority; when 20-year-olds were considered mature adults, and an unmarried, skateboarding 30-year-old would have been a laughing-stock; when white-haired 80-year-olds were regarded with reverence as fonts of knowledge instead of as technologically out-of-step, silly left-overs of a bygone time.
Interestingly, Robert A. Heinlein, who specifically addressed the topic of longevity in many of his novels, got it all wrong (with regards to the effects of said longevity on society as a whole)!
Regards,