Not to be too glib but my mom would ask counter questions like this:
Why is it that we have to trim out nails when they grow? Why not leave it natural?
Why do we remove the weed in between the pavers in our backyard? Why not let it be natural?
The fact is the world around us needs constant work. Our capitalism is no different. It needs constant pruning or it becomes gluttonous. There was a book I think which said most people involved in illegal drug trafficking are barely getting by, most of the income is soaked up at the top. I don't remember the point the bio was trying to make but it feels like that way for any enterprise these days.
The richest people in the US have reportedly increased their net worth by over 1.5T over the course of the last year or so.
How is this sustainable?
US homeowners have increased their net worth like $15 trillion since the start of the pandemic.
Besides, there's this thing called tax incidence and it's not as simple as "tax the billionaires" because it's not clear how that plays out in terms of people's wages or middle class investments.
On the other hand, land value taxes would actually be incident on landowners.
Isn't this basically Entropy? Why do stars fuse and spread out their energy? Can't they just keep it all in? They are going to blow up/die out eventually, how is this sustainable?
The notion I'm getting is that these forces that drive change are bigger than all of us, and they are inherently unsustainable in the larger scale of things, pretty similar to how solar systems are not really sustainable in a scale much larger than us, but not that is still pretty small in a universal scale.
So for your perspective it might be unsustainable, but for the bigger system what you describe is smaller than a grain of sand.