Perception matters when it comes to living standards.

There is a phenomenon known as anchoring, you can read about it in books on Behavioural Economics such as Nudge by Thaler and Sunstein, it shows the effect of an anchor on the perception of statistics. I forget the exact study, but it went something similar to this. "Is the tallest tree above or below 1000 Yards, guess with your estimate" people guessed about 800 yards. If you ask people "Guess the height of the tallest tree in yards" and they guess about 200 yards.

Now, this isn't a perfect analogy, but it works similarly. Each day, we roughly assess our chances of dying, getting sick, or injured, and we mostly assume its rather low. Therefore, if I told you there is a 1 in 1000 chance of dying today, you'd be fairly concerned. Its low, but if every day was like that, you'd be dead in under 3 years. Therefore, you're going to spend the day fairly flustered. However, if a plague is going around, its medieval Europe, and people are dropping daily, if I said you have a 1 in 1000 chance of dying, your perception is going to be fairly good.

Connected to the first point its area-specific too. For example on womans rights, Michael Woods "The Story of China" excellently includes a hefty dose of female literature and if we look at the Qing dynasty as just one example, authors are divided on whether it is a brilliant time to be a woman or an awful time. On one hand, there are impossible beauty standards, foot binding, and seclusion, on the other hand, Woman authors can get published. Similar goes for the Working class in different era's, we tend not to notice our present conditions unless there is something to meaningfully compare them to, therefore it also depends on your own experience. Marx needed to see urban worker classes next to wealthier aristocrat classes. However, it usually takes an alternative being visible for one to be able to perceive change or something different. You can of course question where things like the first revolutionaries come from, but often they have to "invent" or imagine the alternative, or its just the culmination of gradual progress. For example, Americans in the war of Independence somewhat harked back to an imagined time of particularly English virtue, which in some ways harked back to a time before the "norman yoke" etc as a form of contrast between the then and now.

For example, what will people look at our generation for and think is positively ghastly? Eating meat perhaps? Any sort of carbon emissions? Paying for university education? Any sort of Inequality? Or will they think we went off the rails, dangerously modern ideas, multiculturalism? Globalisation? Who knows, but perception is the key to everything.

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(Note: I chose multiple things that could be "ghastly" to maintain some sort of political neutrality)

I try to give answers to specific points:

-Terrible life expectancy

It's important to remember that average life expectancy at birth isn't a good indicator of how long people actually lived. High infant mortality suppresses that number by a lot.

-One third of the world being slaves with no rights, some born into it

Slavery is bad; but there are different versions. Most people think of American style chattel slavery. But in other cultures there was a wider range, with some people treated like animals, and some like parts of the extended family.

-Constant war, if you’re a man, the expectation to fight for your country

Not everyone was expected to fight for his ruler/country. Mass conscription is an invention (or reinvention) of the French revolution and the Napoleonic wars. Of course if public order breaks down you'd better get yourself armed if invaders come.

-Oppression of women

That's not as black and white as media presents it. Most people back in the day were farmers. So as man you had the career choice of becoming a farmer and do hard field work, and also some household stuff, and as a woman you had the career choice of doing household stuff and some hard field work. Not really that much difference. Also, it is important that when your hear a "feminist critique" you actually hear "western upper class feminist critique". This is somewhat based in the idea of the individual, and that only traditional male roles are worth pursuing. The idea that the family unit is important (that is the combined resources of wife, husband, parents, siblings, children are indicators of success) isn't really explored. And neither the informal power woman used to wield. An example for this is the late Chinese Han dynasty. Officially the Emperor was the total ruler of everyone, behind the curtains he was a puppet of various women and eunuchs at court.

-Ridiculously harsh punishments for crimes, like being thrown into an arena to fight in Rome

The key here is that this was for crimes, something people caused themselves. And culture is the key. The people back then just were more used to cruelty. To make a current example, the US justice system is very brutal compared to the Nordic countries systems, yet people in the USA still can be happy. Unless they get caught by the system.

-Disease with no medicine

That did suck. Catastrophies like the Black Death did leave a mark on people and culture.

-Duelling to death over petty things

That was something for nobility. Not everyone had the right to challenge everyone else to a duel.