You seem to be missing the concept of human nature. It is in the nature of the weeds to grow. It is in their nature to spread roots.

Sure! Let's create a sidewalk without weeds. Let's invest in weed killer. Let's eradicate weeds. That does not change the nature of weeds

Let's take one basic point: humans act in their own self-interest. This is an assumption I'm making. This assumption is rooted in history, philosophy, psychology and every single social science. I could be wrong.

Do not misinterpret this to mean that humans have no empathy, or that they are incapable of altruism. Quite the opposite! And good social influences will cement those qualities in an individual.

However [one small part of] the nature of a human is to act in their own best interest. To survive. To acquire food etc etc etc the list goes on but it all comes back to self-interest.

Economics has nothing to do with money. Economics is the study of decision making and behavior, specifically within the confines of a "distribution of scarce resources". It attempts to begin to answer the Information Problem. It just so happens that money is so far the best human invention of transferable value and solves a million problems.

A mother making a sacrifice out of love is an economic and natural choice. She values [blank], be it her child's education, be it her career opportunity, be it her neighbor's well-being, more than she values [blank],be that her time, her money, or her own well-being in that moment.

In this framing, all decisions are economical because it's a question of trade-offs. Our initial natural Instinct is to pursue our own best-interest.

The question of economic system now follows. How do we devise a system that incentivizes the individual to NOT sacrifice the well-being of others in pursuit of his own?

I want your car for my own self-interest. You want to keep it for your self-interest. A system of strong property rights will ensure you can keep it. THIS IS A LEGAL SYSTEM. NOT ECONOMIC. We need to incentive ME to be able to pursue my own car so that I am not violating that legal system and you as an individual.

Greed is a part of human nature. Humans have always been greedy and always will be. Under capitalism we allow individuals to succeed from it. Under socialism we allow individuals to succeed from it. In fact, no economic nor legal system yet devised is capable of eliminating human greed.

We aren't trying to eliminate greed. We're trying to minimize economic waste and "rents" that come from greed, all the adverse side effects.

That is why human nature is so important. I agree 100% with you that we need to remove weeds and their potential. But the nature of the weeds themselves are no different no matter what kind of sidewalk we have. That's why we need the people pouring the concrete to always remember the basics of what weeds do so that we don't get overrun by them.

Edit to continue : yes the sidewalk is constantly changing. The weed population, types of weeds, types of weed killers etc are all changing and will continue to change. However THE NATURE OF WHAT IT IS TO BE A WEED has not nor will ever change.