Can we talk viability vs comparability?

I see this word being used so much in threads that aim to discuss balance issues that I feel this actually needs to be said.

There is a huge difference between a job being viable, and a job being comparable to others in their roles. Whenever someone argues that a job needs help or changes, it's never because we think it's not viable or can't clear content. We all know the game is designed so that anything can clear.
It's because, comparatively, it's lacking in its role.

To say something is viable as a argument is a copout excuse. We all know anything in this game can clear content. Does that mean nothing should be touched on throughout despite it's flaws? Is there a problem with wanting our favorite job to be just as good as the rest?

I just wanted to make this topic, because it's a huge problem in topics where entire arguments are simply tossed aside with a "but it's viable" statement.