The TVTropes Trope Finder is where you can come to ask questions like "Do we have this one?" and "What's the trope about...?" Trying to rediscover a long lost show or other medium but need a little help? Head to Media Finder and try your luck there. Want to propose a new trope? You should be over at You Know, That Thing Where.
Your example is Sealed Evil in a Can, and there are a lot of possible reasons why it's a Justified Trope and they couldn't do anything more permanent than sealing.
If they do anticipate the evil is going to break loose in 1000 years and they provide a weapon or instructions, that's Fling a Light into the Future (under the "because it'll appear when you're long dead" clause).
Wondering why they didn't bother even flinging a light into the future sounds more like a Headscratcher to me. They could be lazy, or maybe their first solution was victim to It Only Works Once. It's so wide open, assuming the worst is just speculation.
Edited by KD
A trope for whenever there's this biiiiig problem the characters encountered encountered ages ago that they deal with for a bit and then proceed to do absolutely nothing about.
Like, say, a great evil that they fought and sealed away, and then they just... sit on it, for centuries or millennia, not doing anything further. Inevitably, when said great evil gets loose again, despite having ages to come up with countless ways to deal with the great evil, they're somehow caught completely by surprise and thus probably are steamrolled over.