Makes me wonder if we hit a high point landing people on the moon, and everything's just going to go downhill for the next few thousand years.
However while the Roman Republic relied primarily a 1-3% annual property tax on wealth to fund its early expansion from 509BC, it had abandoned public assessed domestic property taxes by 167BC at the latest, yet the the Roman Empire reached its height in 117AD. So the point in time at which a country eliminates domestic property taxes on landed wealth in order to maximize the private capture of rent for its political class is unlikely to represent the peak or maximum of the civilization, but rather an initial inflection point, in which case a peak of our current phase of civilization during the 1960s may have been possible, with the inflection point occurring earlier.
When land + labour is the primary means of product, maybe, bu since that hasn't been the case for a while and may not either have been the case in aboriginal territories, I'm not sure it applies.
While talking about an Empire expansion you omit the biggest factor: You expand or grow conquering-stealing-destroying others and the limit for this is geographic, not taxes.
In the particular case of the Roman Empire they needed to communicate all the provinces, and they did that by sea, mainly using the Mediterranean Sea. There was no train, like there is today(which is a tremendous difference as it is several orders of magnitude of difference transporting things).
Once you conquered all the Mediterranean Sea, there was nothing else to conquer, communicating by land was super expensive. Today it is the size of the world, until we get out to other planets.
Also in today's world there are weapons of mass destruction like nuclear weapons. If you try to grow at the expense of others you could get destroyed.
The downfall may have been an effect of the great thing. Pyramids, sweet! There's a drought coming.
I wouldn't go for the moon landing, not because it wasn't great, but it wasn't an all-consuming investment by civilization. There are a lot stronger modern candidates.
That may be so, but it remains as a feat of project unmatched in their history, the one great wonder that survived. I am not sure how many would be able to name the pyramids that came after. There's a huge amount of precision involved and a huge amount of effort required of the culture to complete.
Similarly you could argue that with space exploration we've become significantly more advanced, our probes and instruments can do things the ones in the 60's couldn't. Yet, we haven't sent anymore humans past low earth orbit. In a sense we've advanced, the individual efforts remain great, but our cultural efforts have retreated, the weight of humanity is not where it once was.
What's most going to be remembered of our culture in 5000 years?
There is a good chance that computers will have an uninterrupted existence for the next 5000 years, and we’ll be remembered for creating the first network with 1billion+ users, similarly to how the Romans are remembered for their roads and aqueducts. This is a particularly exciting case because large amounts of data we generate could survive!
In the case that history is largely forgotten, similarly to how we have a heavy fog around knowledge of society 5000 years ago, I can imagine future archeologicists being very surprised to find large piles of fissionable materials. This has the potential to be very sensational in future pop culture.
Either this society continues (and solves huge environmental challenges along the way), or we have a scifi scenario, where a group of humans on faraway planets forget about old Earth, or no one will be able to remember or rediscover us.
It’s really a matter of how bad the ecological disaster is, I can imagine a point where it’s bad enough to cause a collapse of our current civilizations but not bad enough to prohibit smaller less technologically advanced civilizations.