Without knowing the future, I think the best we can do is look at historic trends to give us an idea of what is going to happen in the future. I agree 1800 is an interesting datapoint but probably not that relevant for the present day. Nonetheless, there appear to have been consistent improvements even in the past few decades, and there is still plenty of scope for improvement given almost 10% of the world population are in extreme poverty (about 700 million people).
I think a lot of people answering the survey question:
“All things considered, do you think the world is getting better or worse, or neither getting better nor worse?”
focus on whether their world is getting better or worse, and not the whole world. Most of the improvements in the past few decades have been in developing countries, which have experienced dramatic improvements in standards of living (and developing countries are also home to the majority of the global population). Meanwhile standards may well have been relatively stagnant or declining for many people in developed countries.
If we are talking about whether the world is getting better or worse, expectations based on experiences in India and China are far more pertinent than the recent history of the USA.