If Monks have compassion , which they have , then why don't they show us or show me the magic which they perfected?
It is out of compassion that the magic is NOT shown to you. I am just not finding a right reference for this sutra but I have read that the Buddha once admonished a monk showing magic to lay people saying;
Monks, attracting people to Dhamma by showing magic is like a
prostitute attracting clients by pulling up her skirt.
This is a very important thing to understand, that people should come and follow the Dhamma for all the right reason and that right reason is 'to end suffering'. If monks would have started showing magic to preach Dhamma, the whole purpose of Buddha would have defeated because people should have come to Him to gain powers.
Because if they show me some real magic then my belief on the permanence of physical laws will vanish.
No that will not happen. If I will show you I can fly in the air, you won't think that gravity is impermanent, your mind will make a new idea about some physical law that was helping me levitate.
Mostly I believe in the permanence of physical laws like no person can go through a wall or that what will go up will come down or that if slapped I will be in pain...and so on... This belief in the permanence of physical laws came into existence in my consciousness as given the exact causes exact similar things were happening since I took birth. Real Magic will break that belief.
This is the same question you were asking about Gravity. No, with seeing real magic your belief in permanence will not fade, you will, in fact, start believing in some other strange things, and this time they will be strange. And further you will start craving for that magical power instead of the Dhamma.
And once I my faith in the absoluteness of the laws of Universe is broken I will start questioning the reality. I will start asking what all in not permanent ? I will start wondering if Universe doesn't work by those laws then which laws ? Should I conclude there is no permanent law ? Desire for investigating will develop as desire of investigation is one the 7 factors of awakening.
You have already known that things around you are not permanent, is that not enough for you to develop the desire of investigation into Dhamma?
Further, I will state that the laws of physics that you are pointing out is not something you can ascribe the quality of being permanent or impermanent. I will like you to read the book called The Science Delusion nu Rupert Sheldrake. In that, he has shown that even the speed of light that is held constant is in fact not really constant. That book has answers you are looking for and not levitating monks.
Therefore for the sake of humanity why don't Monks show magic and convert the lay people into Buddhism?
For the sake of humanity, they are NOT showing the magic. That's not how you convert people and convert into what?
In short, why real magic is not a good illustration to make one realize the Truth of impermanence of all physical Truths.?
You are misunderstanding 'physical truth' with the description of processes. The physical truth like a gross body can be clearly seen as impermanent and the truth like 'law of gravity' is a description of the process which itself is proof of impermanence. The law is the proof you are asking proof of the proof.
Real magic will lead you astray, it's for the street shows not for the Dhamma.