Should I buy 2015 13 inch MBP or 2016 13 inch (non-touch bar) MBP
The crux of your question/decision seems to lie in the following assumption:


  Since Apple discontinue product after 5 years, so if I purchase 2015 model now I have 2.5 only years before it is discontinued. And after that if any part need replacement I will not be able to do that even if it is under apple care after 2.5 years.


However, your assumption is wrong.

I refer you to Apple's policy on Vintage and obsolete products, which outlines their hardware/parts support. 

In summary:


Vintage products are those that have not been manufactured for more than 5 and less than 7 years
Obsolete products are those that were discontinued more than 7 years ago.


Note: Bold emphasis mine.

In essence, the 13-inch MacBook Pro (2015) model is still being manufactured and has not been discontinued yet. Apple's warranty provides global coverage, and if you purchase Apple Care then you will be covered for the whole of that Apple Care period.