Why do people stop playing? Because the game is not renewable. It is a straight line from 0 to 1, reached once you've got a few million LP. After that point most have nothing more to do, discover or build.
We're immortal, we never age, we don't reproduce, and we don't need to learn any more skills after a certain point. We don't have challenges either, and we don't need to adapt to the weather or to random phenomena. So most of the things that make the real world change and transform in interesting ways is absent in haven, at least for the moment.
Real life doesn't have an end, you can always explore places no one has been before, learn more complicated things, build more expensive structures, and craft more unique and refined items. If haven had that it would certainly give people something to look forward to.
As endgame boredom is by far the most common complaint, there's quite a lot of general suggestions that attempt to fix it:
Endgame skills
viewtopic.php?f=48&t=50986&p=674585
Endgame activities
viewtopic.php?f=48&t=49571
Megastructures that make a kingdom work weeks to build
viewtopic.php?f=48&t=56780&p=731061
Seasons
viewtopic.php?f=40&t=53114&p=704207
Dynamic politics
viewtopic.php?f=48&t=52762&p=699618
Dynamic time
viewtopic.php?f=48&t=50162&p=665164
Ocean content
viewtopic.php?f=48&t=52726&p=699035
Kingdom objectives/quests
viewtopic.php?f=48&t=51612
Interesting geography
viewtopic.php?f=48&t=51972&p=687466
Forensics/archaeology to make the world less dull
viewtopic.php?f=48&t=48265&p=630365
Tools to let player actions leave a more lasting effect in the world and create permanent lore for everyone
viewtopic.php?f=48&t=51049&p=675513