Unable to boot up my Mac. Getting a flashing folder with a question mark
Take a look at EveryMac's Retina MacBook Pro page here. There is valuable information regarding compatibility (or lack thereof) of SSDs between different models, options for 3rd-prty upgrade offerings and lots of stuff you generally need to know about SSDs for Retina models.

Dealing with your actual problem at hand, SSDs aren't difficult to test, and any decent technician should be able to diagnose the issue. If your technician says it's died then by all means get a second opinion but SSDs have the capacity to fail like any other electronic or mechanical component.

If you're an EU resident you may have a case for a free replacement from Apple as EU-wide consumer law forces companies to offer a 2-year minimum warranty period. Getting them to honour a warranty may not be easy but it's another option. You never know, you could get lucky and find someone sympathetic to your plight who's willing to swap out the defective SSD.