Even scarcer technologies may emerge in the future, but their value will be challenging to replicate.
Here's the thing. Perhaps something more limited than bitcoin may exist in the future, but the question is whether it will be attractive enough, whether it will generate the demand that bitcoin has. As it has been pointed out, for something to be valuable, it not only has to be scarce, it also has to be in demand.
Yes, there are many altcoins with even more scarce supplies than bitcoin and it is not difficult to create altcoins with such fixed supplies.
Supply is also an important factor but it is meaningless if it does not attract people and there is no demand for it. So don't focus too much on supply, but pay attention to the use cases and applications that the asset brings.
Yes, the essential properties, I would say. Decentralization, security, and autonomy, mainly.