Another motivation for the multiverse is this:
Ibliss:"Physics, as it is practiced now, is necessarily incomplete. A fundamental theory needs to postulate certain fundamental physical quantities like the physical universe, spacetime, fields (or strings) etc. and fundamental laws of physics. These quantities are themselves beyond the realm of explanation. If they can be explained then that would only mean that there exists a more fundamental theory, with less fundamental quantities."
I do not agree with your statement below, for two reasons.
a) If indeed "fundamentality" is characterized by the number of basic concepts/quantities, such that thew more fundamental the theory the fewer such concepts/quantities, it is not necesarily that a more fundamental theory cannot make use of already explained concepts/quantities. Even if such a series of fundamental theories convrges in the sense that in the end there may be a theory with no unexplained quantities, this does not mean that such a theory does not have fundamental concepts.
b)Practical experience, so to speak, shows exactly the opposite of your statement. the more fundamental the theory, the larger the number of such necessary concepts. Think about statistics and quantum mechanic, or even better, about Newtonian mechanics and general realtivity. In classical mechanics, geometry is a given, while in general relativity it becomes one of what you call fundamental quantities. So for (at least) the time being, the trend appears to be opposite to what you state.
Ibliss: "Clearly, the only possible way in which everything can (in principle) be explained is if there are no fundamental physical quantities at all."
I am not so sure about that, say for the reasons above.
ES:"Suppose that there doesn't exist a physical universe. It could be that only mathematical worlds exist. That would mean that all possible mathematical worlds are ''as real'' as this one."
Why this kantian aproach? Besides the fact that it is far fetched principially, what does such a concept bring as useful knowledge? If I were to follow Kant's conclusions, in the end each of us is a walking asylum, so to speak, and what matters is only what one believes it matters. In the case of your example, you end up with a sort of mathematical mysticism, where only what one believes matters, and there is no "objective" (that is experimental) way of testing one's belief. How exactly would this help with our knowledge?
I am trying to imagine now, in our present "asylum", what would happen if what you say were right. Well, you would have instead of human beings some sort of kabalists making more or less preposterous claims, and worse even, fighting for the power of imposing their system of beliefs.
Hm, this sounds very familiar, so maybe I am not imagining it and you are right
