Living Story and Elder Dragons
Firstly your understanding something wrong. Arenanet has actually said that it hasn’t decided how to release expansion style content at this stage, weither they go with a traditional expansion set or find a way to incoperate it into the LS. The LS team is only a part of the GW2 dev team. There are alot of other devs working on longer term projects that they just arent ready to tell us about.
Secondly, they have said the dragons and the PS will be back. The LS is designed to set a narative to the world. Its also to show us that there is alot more to the world than just a bunch of EDs. The LS has reverenced the EDs a few times already and will probably do more so going forward. Any new content or PS involving fighting the EDs, I suspect, will be perminant though until Arenanet decides to tell us it only speculation.