I really don't like Laios's argument because Marcille is literally right; she's not FORCING people to live a long life. People can simply choose to kill themselves if they feel like they've had enough. And I think MOST people would choose to live a long life if the "see all your loved ones die before you" part was taken care of.
Also, a big problem with Thistle's kingdom was that he just fucking sucked at making people happy. They were stuck in a tiny village where you couldn't even eat good food. Marcille was asking for Laios and gang's help specifically so that she could create a dungeon actually good to live in. The dungeon is, realistically speaking, non-distinguishable from the surface. It has a sky, clouds, oceans, sunlight... It's just another dimension except someone gets to create all the rules for it.
(Also obviously this is all speaking from the POV of Marcille and what she believes in. We as the reader know the Lion is lying and he just wants to destroy the surface)