I have rarely seen a car becoming unusable after 15 years. I have seen taxis with more than 400,000 KM on the odo. Not that they never had issues, but they are pretty much usable if you replace parts.
The 15 year car rule is only applicable for New Delhi NCR Region. You still need to obtain fitness certificate for vehicles every 5 years after the 15th year, but they need not be dumped as long as it passes the evaluation.