The problem is regarded as serious

Example Sentence from Longman Dictionary:

The problem is regarded as serious.

Without any doubt, as is a preposition here. So how could serious, an adjective, serve as the object of as? I was always told the object of a preposition should be N or NP.

Or I don't regard as serious as a PP. I regard serious as the subject complement. In this situation, what is the part of speech of as? A conjunction or still a preposition? Or just a meaningless word that connects is and serious?

It's valid that a given adjective X can be used as a noun; it will mean "any of those that are X."

Example:

We take care of the poor = We take care of any of those that are poor.

The problem is regarded as serious = The problem is regarded as any of those that are serious.