But people dismissed these warnings, as they came
from the (now free) Eastern German media

As an East German who participated in the demonstrations (17 at the time), your version is completely and utterly wrong in the main part. Yes, the reuniuon was painted in a way too nice light. No, we East Germans did NOT vote for it because we were mislead - the vast majority of East Germans would have voted for it in any case! There was no alternative.

The vision of being stuck with a useless currency without any value, in the middle of crumbling infrastructure (and whatever problems US infrastructure may have - and I lived in the US for a decade - it's not even close to East Germany, and yes, we had plenty of lead pipes too, in my own house for example), incapable of traveling anywhere (no money), a dead-end society. Our environment was a huge disaster!!

Without reunification everybody who could would have moved West. You may argue that has happened anyway, but I would say not nearly as much as what would have happened if the GDR had remained.

As far as "cost" - this is a somewhat silly argument on the level of an economy. For me, yes, I have to look at my costs, same as for a business. But in an economy somebody's cost is somebody else's income! The US does a lot of "socialism" and planned economy via military spending. Germany did the reunification. I think the German money was well-spend in comparison. Okay sorry, that wasn't supposed to be an argument about US military spending, I have no idea about its overall effects, but I know about the effects of the German reunification.

The environmental cleanup alone was HUGE, you have no idea (it seems to me). I lived next to a very large chemical fiber factory (where I learned too), such filth, huge mountains(!) of ash nearby form the (horribly dirty) power plant, the river that ran by without much life and you didn't want to touch the water. Today: The water is near perfect, the ash-mountains gone (there is a big new modern factory), the power plant modernized, everything is clean. Yes there are a lot less people in the area, but overall I consider it a huge plus.

The depopulation problem is not actually alone that people moved out of East Germany: Quite a few larger cities there are gaining. A big part of it is people moving to cities. Same reason why some areas in the US (Bay Area, where I lived) are gaining, or Munich or Berlin in Germany, or Moscow and St. Petersburg in Russia, while villages suffer. East Germany had been the less populated and more rural part of Germany before too!

So no, overall what they promised about the reunification was not all that oversold. Some individuals may disagree, the majority though most definitely does not regret voting the way they did.

However, your misrepresentation of what happened does remind me of what is going on now. Trump is the work of Russian hackers! People are mislead! As someone who feels actually pretty left (at least socially very much so, and when it comes to risks and rewards distribution in society), I am disgusted by what I have to read from "my" camp. Zero reflection, zero analysis. Trump brought out the worst, that is true! I see a lot of it in many of those arguing against him though.