Samsung - Samsung quality issue

I bought a Samsung A** mobile on * October **** for my uncle. After * Year and * months of Normal usage, one day while attending a call mobile started rebooting automatically and it kept restarting and shows samsung logo. This happened multiple times. I gave the mobile in Samsung service center, after a day they told its booting IC issue you have to change the motherboard and it costs Rs ****. Reason they mentioned is over usage of memory. Firstly, its not overused and I don't understand how memory usage resulted in Hardware issue.
I asked how they identified the issue, reply was since it is not booting so we need to change motherboard and they don't have any equipment to identify the faults.
Few thing I don't understand here,
*.Mobile never fell down not even once.
*.Motherboard completely sealed. I cant touch it and never gave in outside shops for repair.
*.Does Service center just replace parts? Why there are no equipment's to identify issue?
*.Replacing motherboard means buying new phone. We know that new Phone will work. is that the only solution you have for mobile repairs?

Definitely there is a quality issue in the product. With normal usage and no physical damage how come motherboard gets a problem?
Replace my mobile with quality one or refund my money. its **,***Rs mobile.