User blog comment:ZerefMidnight/Gray Theory/@comment-7418318-20150407134901

"First, in Future Rouge's timeline why didn't they destroy the Eclipse Gate or destroyed when Achnologia became active sometime after frosch's death. Considering the fact that future Lucy told of what would happen if the gate would open."

The reason why the Dragons came to invade through the Eclipse Gate was because it went out of controll. They might have been conducting research on how to use it savely and thus didn't destroy it.

Or they might have left it intact due to the dangers of it's destruction. When Motherglare crashed into the Gate it had already expended at least part of it's stored energy to establish the time-tunnel. In F-Rogue's timeline the Gate might have most or all of it's energy still stored, which could have led to a minor (or not so minor) disaster if the Gate was destroyed.

There's also a good chance that while Hisui and Arcadios believed Lucy and thus let her close the gate, the King might not have believed Lucy and intended to re-try the Eclipse Plan once the conditions for the time-travelling were met again, but Acnologia struck before the next suitable Lunar Eclipse came about.