Even if you could point to some other force, field, or whatever, you're only trivially further back in the causal chain.
I don't think this necessarily follows. If you follow Krauss' work, there are some subtleties to his argument. I agree that eventually, when you get down to it, there will likely be a few axioms you will need to start from. The same is true with geometry. You can't even define a triangle without defining a few axioms first, so how can we be expected to define all of existence? Once we get past that though, you will realize there are very few axioms required to explain existence, and none of those axioms "require" god.
Now it follows from Krauss' argument that if we are to believe in quantum mechanics, the natural conclusion of such a theory is a universe similar to our own. So why not analyze the postulates of quantum mechanics and tell me if any of that requires god. I think you will find the postulates completely mathematical and logical and without the need for god. Therefore, when asking the question about why we exist instead of not exist, you might as well be asking why do mathematics and logic exist? Are these concepts eternal, do they exist independently of a physical universe? Does logic and reason require god in order to be true?
There are some subtitles to Krauss' argument that require some explaining. Here is my best explanation.
What if all that is required to derive all the laws of nature, including quantum mechanics, including the multiverse is the symmetry of empty dimensions? A simple example of this is what we get from SO(10) symmetry group. This would be special orthogonal rotations in 10 dimensions. Note, I have not defined these as spacial or temporal dimensions yet, they are just degrees of freedom. Everything looks the same, no matter how you rotate these dimensions. This is a symmetry group. It happens that this symmetry group can result in the symmetry that we have today, namely SU(3)xSU(2)xU(1) which is what defines the Standard Model of particle physics. Now, all we have to do, is show how the symmetry of SO(10) can break into the Standard Model. We have done this with computer simulations and our the results look astonishingly similar to what we have today in terms of matter density and physical laws. Now SO(10) is just one possible starting point. In reality, there can be an entire landscape of possible starting points and this would be referred to as the vacuum landscape. String theory puts some lower bound on the number of possible universes to be around 10500 but the number could just as well be infinite. The multiverse, every possible universe, arises naturally from the quantum vacuum, or the physical nothing if you will.
So why does this happen? Why does a ball roll down a hill? We know the answer to this. It is because it is following a path that minimizes its energy. Why is there a lower energy state to the universe? Because if we wait long enough, fluctuations will result in such a lower energy state. Why are there fluctuations? Because of quantum mechanics. Why is there quantum mechanics? Because of the multiverse. (Search for Sean Carroll and Born Rule and multiverse) Why is there the multiverse? Because the multiverse is mathematically equivalent to physical nothing. Why is there physical nothing instead of absolute nothing? Because if there was absolute nothing we wouldn't be around to question it.
In any derivation such as this, there will necessarily be axioms. One of the axioms will be to define the vacuum state, in other words, define the physical nothing. What is astonishing is that is all that is needed really. Once we have that, the rest sort of follows naturally. The physical vacuum is the closest to nothing that nature allows. You can't have less than this because quantum mechanics forbids it.
Now if you analyze my scenario carefully, you will find a gap. How do we get the physical nothing in the first place? Maybe that physical nothing is eternal. Maybe it has existed for all time. Maybe existence wouldn't be well defined without it. The point is that I don't just immediately come to the conclusion that god did it. There could be many explanations that do not require god. If this is the god of the gaps argument, science has now reduced god to nothing more than a quantum fluctuation on a vacuum landscape. A fluctuation that would happen spontaneously, and is required to happen thanks to quantum mechanics.
So is this a viable theory? I think so. Are there gaps in the theory? Absolutely. Do those gaps immediately imply that god must be responsible? Absolutely not. Therefore, is god necessary? Absolutely not.