A thousand years ago puts you in the era of King Canute who ruled Denmark, southern Sweden, England, southern Norway, the Venetian Republic taking shape, the Romans ruled from Campania to Armenia, you could see the Islamic Golden Age, you could see Great Zimbabwe, you could see Song dynasty China inventing gunpowder and protoindustrialization, and the middle of Heian Japan. Who knows what a thousand years in the other direction could get you.
The thing about those events are that theyr're all around the world. Even with 1,000 year life span, by virtue of being human you are not
going to experience all of that, but if you're lucky, just bits and pieces of it and only if you're in the right place at the right time. During that thousand years, most of the progress has been compressed in that past 500, with much of it exploding
in the past 100 years. There is no guarantee this pace will continue over another 1,000 years.
It is also possible that long life spans would cause progress to come to a grinding halt. I personally believe the Black Moon Clan's raison d'etre in the manga is 100% correct; removing death as an obstacle would stagnate collective
human progression severely since outside of those self-directed enough to achieve regardless of external circumstances, there would be no impetus to improve. (It's their horrid actions which render them irredeemable. Wanting to reintroduce a fear of mortality it no excuse for murder, terrorism, infiltration, and invasion.) So as a societal level, it might not be a good thing. That doesn't mean I don't want it for myself, though.
Since your spouse can also live for a thousand years, you can still have only one spouse if both of you choose to.
There're actually another big complication concerning this...your spouse might not have been born at the same time as you.
Imagine, for instance, that you are a Millennial, which means you'd have been at the peak romantically when Crystal Tokyo came about. So you're among the first "immortals."
Then the Gen Z/Zoomer crowd comes along. As a Millennial, you would normally see these people as your kids and the next generation...but all of a sudden, these are actually viable partners. While it may seem creepy at first, this really isn't too bad...out of 1,000 years a 20-40 year age gap isn't that bad.
But then eventually, you find yourself single for a 150 years and BAM! You meet a perfect partner. The problem? The partner was born about 100 years after the founding of Crystal Tokyo. (Let's assume the immortality begins at 20 so we don't have 30 year old babies and such). That's nice....except you can only spend 850 years together. It's not that bad of a deal...except some people could conceivably end up getting married or remarried several hundred years into their lifespans with people whose lifespans are significantly longer or shorter.
As I wrote in the other thread, the big romantic problem isn't going to be cheating or necessarily opening up relationships because there's a perverse incentive to not
fool around. No, the threat of death to relationships is now increased substantially, since you have people who could die in decades or less pairing with people who won't die for centuries
. That means having one spouse will only be reserved for people who are lucky enough to fall in love with people in their relative age group.