There is no reason other coins could not update their code to become resistant to quantum computers since it is not like they are going to appear out of nowhere the devs will have time to adapt and update the code of their coins.
Agree. If the development team of a specific cryptocurrency, is a very active one, then it shouldn't be a matter of time before it gets upgraded to improve their security against quantum computers. The ones that might not upgrade though, would be those altcoins which are pump and dump, as well as have an inactive development team and community.
In my own opinion, I think that the first coin in doing the improvement would be Bitcoin itself and then the vast majority of altcoins will follow. It would be quite interesting to see what effect quantum computing would have on the security and stability of those coins which have not made the switch yet.
Nevertheless, the altcoins that are prone to survive are those who can maintain active development and a solid community over the years. Just my thoughts.
