That's not the definition of wealthy. Wealthy is a relative term. It's meaningless to say that a lower middle class person today is wealthy just because people 500 years ago could only dream of the things they have. If other people in the society they currently live in have a million times more, they are not wealthy.
I'm sure you understand how capitalism works. If you do, you should understand that not everyone can be an owner, someone has to do the actual work.
That's like saying anyone with legs can be a professional runner. In theory yes, but not in practice, not in a meaningful way.
Defining poverty as bottom quintile is also useless, yes. Defining poverty by comparing to the average or median income is the most useful definition. Someone earning less than half of average, for example. That tells you how well they can afford the limited resources of any society, as those track the average purchasing power.
I'm sure you understand how capitalism works. If you do, you should understand that not everyone can be an owner, someone has to do the actual work.