Once the best allocation of currently existing ressources have been implemented, then there would no longer be any growth.

The demand is dynamic though. Even if you have right now an ideal implementation for resource allocation that won't necessarily match future demand.

It seems far fetched to assume that people's preferences will change just in the right way that there is always increasing welfare from the same basket of goods.

As I understand it, free markets determine preferences based on price. However, the price someone can buy something for includes both their desire to pay and their ability to pay. Therefore, I'm not sure that price is always a good metric for allocating resources.

Do resource zero-sum, free markets still reliably experience growth from the perspective of someone who doesn't view price as a good measurement of value?

If so that would potentially be useful because it could allow more progressive leaning and more libertarian leaning people to find some common ground.

If you pay for something it means that your value for the money is less than the value of the satisfaction you believe you will get out of it. Money does not magically create an equivalence class of value across all items of the same price.