iPhone 7 suddenly died.

So, if it was in your possession by 17 May 2017 then you are in a stronger position to successfully argue that the phone was not damaged or tampered by unauthorized third parties.

What you won't be able to do is purchase AppleCare+ for it (if available in your country), since it needs to be activated within 60 days of purchase AND the device has to be in an undamaged/operational condition. AC+ gives you two "accidental damage" incidents to use whereby you get a new replacement for a reduced service fee.

To investigate the above, visit this link, type all that is asked for and see if it was indeed brand new or not: Check Your Service and Support Coverage - Apple Support

For an iPhone, the support coverages are:

Default - One year from date of purchase recognized by Apple*

With AppleCare+: Two years from date of purchase recognized by Apple*

*Unless you are in the EU and other durations apply.

Check the full coverage information, subtract 365 days and you will know when the device was originally purchased. If it wasn't in your hands then, it wasn't new when you finally got it.

And if it turns out it wasn't new, then you have no idea of its past service history cause it wasn't in your possession.

Instead of being upset with Apple, you should be upset with Flipkart who obviously didn't sell you a new device, rather they probably do repairs on iPhones and then sell them as if they're new devices

Where did you buy it?

Maybe they didn't sell you a new phone?

Maybe it was "refurbished" by them before they shipped it to you?

I have no idea, that's up to the Apple technicians.

I don't know that online store, I don't know if they are an authorised Apple reseller, or if they sell discount "refurbished" phones.

You can escalate, till you get with the head honcho for the country/region you are in. Then the decisions become final.