Defunct coins are not always totally gone at all times. They are open source as you might imagine and because of that their code gets studied and when there is a new innovation the new ones add that to their system and they simply just use that on top of their already done blockchain. When you have a base blockchain that you wrote that has some innovation plus you study all the defunct coins and see what they did wrong and why they are defunct you can always get a lesson from it.

Sometimes it is a problem with the idea, sometimes it is a problem with the code, and sometimes it is just they didn't know how to do marketing and people didn't simply hear about it or didn't liked it. So, defunct coins are actually quite important part of the crypto world if you ask me.