Another way this system could greatly promote playing, keep worlds alive longer and give people a massive sense of accomplishment would be to make some of the accountbound achievements only available for 1 world.
Would probably be related to newly implemented and thematic things, like this world would have been a great 1-time achievement for mining a certain number of gem boulders.
Having some exclusive rare hat or emote on your account for stuff like this would add a major sense of value to your playtime, tasks could be made sufficiently long to keep players playing later in to the world.
Also, these tasks could be implemented part way through the world as new curios and content is added, so even if the new content is kinda meh it could add a sense of value to them by making the study, eating or foraging of them coincide with a 1-time achievement.