Coins of over four years now might have been silent doesn't mean they are dead, they might be having the issues of maintaining their project and keeping it updated and which is one big responsibility of every project to be able to keep up with the competition in the market in other not to be left aside. It is also one thing that the proper functionality of any project is that the price must be on the constant change with the market, not being stagnant.
Coins/projects that are recently launched and are doing well might be because of the new technology that has evolved and changed the view of things from the old one. That is why only the coins that are constantly working on their project and keeping updated are the ones that are still surviving.
I will say, if the coins of 2018 doesn't keep up working on their project(s), they will drop out for the new coins to take their place, while the coins that has a working team keeps up with their own position, moving with the trend in the system.