If you think about it, all our destroyers are Project 15 and its iterations Project 15 A and Project 15 B. Kolkata and Vishakhapatnam classes are almost identical.
Project 18 destroyers are going to be 13000 tonne behemoths. Maybe we'll see a Project 18A and Project 18B after the initial flight of 6 Project-18 destroyers.
Arleigh Burke class destroyers also have wide variations in between them. They were built in different flights. Just because they are called the same does not mean they are the same.
Look at JMSDF, PLAN, etc. Every navy is doing this. These designs are evolutionary.
The Project 18 will be significantly different, but it is a positive step up. And when the Project 15 were conceived, we were inexperienced in destroyer making. Our weapons systems were not mature. We could not have possibly made a 13000 tonne destroyer back then. Everything is happening in a logical progression of capabilities and complexity. Things could not have been better.