I disagree with your analysis (which seems a common one) in that you perceive shyness as a problem or a negative character trait. Where as introverts are happy with who they are and thier social interactions. I don't think shyness should have such negative connotation?
That's not my analysis: I''m working from the official definition which is generally accepted by the psychological community that Shyness is based around a fear of negative social judgement causing a lack of confidence with strangers. That's why my post says things like "is defined as" and "is considered to be". What I think is irrelevant in the grand scheme of things.
Shyness is considered a trait though, which my post doesn't make clear.
It is generally agreed that if a person self-limits their activities based on fear then it's negative for them.
That said, if someone is shy and has a fulfilled life then there is no reason to force them to change; the problem is simply when it affects them negatively.
Introversion isn't based on fear, but a preference for low stimulation. In fact there's evidence that there is a neurological difference in the way the brain processes incoming information which can be helpful in some situations, but in the world we live in can feel overwhelming. I've heard it described as a "social battery" which goes down fast in social situations.
There are also separate issues like High Sensory Sensitivity which is connected to the amount of information that an individual can filter and process; people with HSS don't filter sensory information so they are often acutely, even intensely aware of their surroundings. This can lead to avoiding social situations because they not only are aware of sounds but also the emotions of people around them and when you are subconsciously aware of the emotions of even a dozen people it's a bit overwhelming.
On the other side there is Social Anxiety which is defined as an overwhelming fear of humiliation in social contexts and which is considered a mental illness or a mental disorder. ETA: it could be argued that a lot of what we call "Shyness" is in fact a manifestation of SA.
