The only Roman technology that was ever 'lost' (that I'm aware of) was a kind of concrete that could set under salt water (if you've ever worked with cement, you know what a wondrous thing that is). But it wasn't really a technology that was lost, but a resource that was used up.

Near Rome there was a pile of sand mixed with volcanic ash; it had a lot of alumina and silica in it, but once it was gone, no one knew how it had worked. By the 1860s chemistry had gotten to the point, that chemist could work out the formula and portland cement was born.

It's kind of a half and half answer. Most of the stuff fell into ruin because of disuse and no repair. During the fourth century in the west, lots of people fled from urban areas because they were no longer sustainable due to a loss in the trade economy.

How did nearly all Roman inventions become such as concrete, bridges and archways, working road systems, civil law all vanish? Wasn't the Roman system meant to "Romanize" the conquered areas?

Thing is...none of that stuff really vanished. A lot of ancient architecture still exists today. People kept constructing new buildings throughout the Middle Ages, so I don't think everything was completely lost. I think that's just the dismal revisionism you get from decline of Rome historians.

As for that last point. You're right, and it did. However, what it meant to be "Roman" changed throughout the 5-7th centuries though with the rise of the post-Roman kingdoms. The Goths and Franks kept a lot of the same infrastructure from the Romans. The Goths had a senate with nobles, the Franks introduced Salic Law (more common law than civic, but the point is that nothing completely went away), Franks inhabited Roman cities. "Barbarian" culture started to meld with "Roman" culture, and I think to someone that might be used to the classic idea of Rome, it doesn't seem the same.

If so, how did everything crumble so quickly?

Again see my last point. But I should also point out that when people left the cities, like Rome, that left a lot of brick and marble to be torn down and scrapped for something else. Hyperbolic, but one could imagine that half of the vatican was built with the same stones of an old temple.

Also don't forget the empire survived in the east!

A question but also an observation I would like to hear the thoughts on from someone more learned:

Would not the breakdown of a centralized state render most of these amazing things impractical? As in if the Romans had to import thousands of tons of stone from overseas territories to Italy to craft that architecture how can we expect some small successor state to be able to manage the same? They have their own things to worry about than buying a shit load of stone to make concrete and make fancy archways or to go off and conquer the lands to get it 'for free'. The same would extend to really everything else.

That's kind of the thought process I've had for a while concerning Rome's fall and their less than impressive successor states; how true is that conjecture? It just seems unfair to apply the same principles from a trans-mediterranean empire to a small successor state of few hundred square kilometers or something.