I guess we can add Counterparty (XCP) to that list. It had its moment, but with the arrival of Ethereum and other blockchains in which you can create and move tokens in a cheaper and faster way, it was displaced and abandoned.
Exactly. Cryptocurrencies come and go due to the evolution of technology. It's a matter of adapting your crypto project to the latest trends in the industry or be left behind. With a blockchain platform like Ethereum capable of doing so much more than just "tokenizing" assets, there would be no reason to "reinvent the wheel" on an older blockchain like Counterparty. Even if the XCP cryptocurrency stops trading at mainstream exchanges, the Blockchain network will always be there as long as there are people supporting it. Anyone can work with the open source code in order to improve the existing project or make a new one from scratch.
Remember, it's not about crypto but rather Blockchain technology. As long as people in the mainstream world are able to find real use cases from a specific project, it'll last a lifetime.
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.
That's certainly true, mate. While anyone can continue working on the code to improve the project, it'll be quite challenging to restore popularity on an old cryptocurrency. People often associate crypto names to a brand, leading to loss of trust if the old project becomes abandoned for a long time. This leaves many investors "holding the bag", as their coins become worthless (due to lack of exchanges trading it). A new dev team could try to revive the abandoned project with the old name, but it's often best to start anew in order to attract as much people into it.
What really matters is Blockchain technology above anything else. We can learn from previous cryptocurrencies' mistakes in order to create better versions in the future. Many features from good-old coins like Primecoin and Namecoin can be adapted into other Blockchain networks in order to make crypto land a better place. Before decentralized domain names became a reality on ETH, Namecoin was the pioneer of this breakthrough innovation. All of this tells us that the underlying cryptocurrency might die, but its technology will live for the foreseeable future. Just my thoughts