First of all, fine tuning is a fact which is not denied by anyone that I know of. The fact that a number of parameters which define the workings of the physical universe are at the goldilocks value is not disputed. The fact that the size of the gravity force is tuned to one part on ten to the sixtieth is accepted by both believers and atheists.
This begs the question. Actually it begs two questions. First, why does the nuclear strong force even exist at all? and Why does it have the precise value needed in order to have the ideal distribution of elements in the universe?
To these questions, answers must be proposed. One answer is that they have these various parameters (gravity constant, nuclear strong force, nuclear weak force, amount of matter and antimatter, etc…) have the value they have because there was an intelligent and powerful Creator who both created them and gave them the value they have so that we could exist.
Atheists, obviously, do not like this answer, but they have a problem. They have literally no other answer. Let me be a bit more specific. They have no other answer for which they have any evidence to support that answer.
When one making an argument is in a difficult situation like this, they have two possible paths. 1. They can change their opinion (and many atheists have–becoming deists or theists) or 2. They can come up with an ad hoc hypothesis. An ad hoc hypothesis is a proposal which is created, not because of any evidence to support it, but in order to counter another argument which does have evidence. An ad hoc hypothesis is one which no one would propose if they were not trying to defend a particular pre-conceived conclusion. For example, in a criminal case, if there is only one suspect, a defense lawyer can propose the existence of an unknown and unidentified perpetrator.
This is what atheists do in this case. So… They created the multiverse theory. This is the completely unfounded theory that there are an infinite number of universes, an we just happen to live in the lucky one. There is not a shred of evidence to support this theory. It is completely ad hoc. One problem with this theory is that, on the face of it, it is an extremely unbelievable theory. Therefore, some resort to an even more desperate measure, which is what you mention above. They say something like this: “Right now we do not have a working ad hoc theory to explain the fine tuning, so our ad hoc theory is that, eventually, an ad hoc theory that will work will be discovered.” Like I said, this is truly a desparate measure, but, as you can see, many atheists resort to this. This is the ultimate example of circular reasoning. I know X is true. There is no evidence X is true. So, I assume that eventually evidence will arise that X is true and in the mean time, despite the fact that there is literally zero evidence that X is true, I will choose to believe it anyway, despite the mountain of evidence that X is not true.
This is the situation as I see it with fine tuning as evidence for a powerful, intelligent Creator. The opponents are left with and ad hoc hypothesis about a theoretical ad hoc hypothesis. I prefer to go with the conclusion that the evidence points toward, which is that there is a powerful, intelligent Creator. In situations in which there is not a mathematical-like proof, reasonable people choose to believe the most reasonable conclusion. This most reasonable conclusion is that there is a powerful, intelligent Creator.
So, you make the point “Maybe there will be some sort of new physics that can explain creation.” True. Maybe so. But extremely unlikely. And we have a perfectly reasonable explanation right in front of us, which is that the universe was created by an intelligent and powerful Creator. I will go with the reasonable rather than the circular-reasoning generated ad hoc speculation.
On the other question, believing in something we cannot see is a common-place of human existence. When we see an effect which we cannot explain by visible things, we propose an invisible thing. If this invisible thing explains the observation well, we accept the existence of this thing. For example, we cannot see electrons, but we believe in them with great confidence because of their fantastic explanatory powers. We believe in human consciousness. We cannot see it, but we can see its effects, so we believe in it. We believe in neutrinos. We cannot see them, but we see their effects, so we believe in them. Until about 30 years ago, we believed in atoms because of what proposing their existence was able to explain. Then in the 1980s atoms were observed in an experiment for the first time. Actually, our belief in atoms was not increased by observing them because their existence was so well shown by their effects. It is the same with God. I cannot see God, but when I propose the existence of God, I am able to explain many things I could not have explained otherwise. For example, the existence of fine tuning. For example the existence of moral truth. For example the existence of anything at all, for that matter.