Two things:
First, you're somewhat overthinking this - Toady first started with the "Losing is Fun" tagline partly because there are no win conditions, only lose conditions, like most city-manager games (and people ask the same stupid questions they asked of SimCity, like "why do you play if you can't win?"). Further, you are meant to not only lose, but lose and then send an adventurer in to explore your fallen fortress later. Creating successive fortresses that build upon the history of one another, or adventurers whose stories intersect and having ongoing worlds was part of the original appeal the game was meant to have, so losing in one game mode wasn't a reason to stop playing or boot up a new world.
Second, this game has no tutorial. You were meant to learn through trial and error before the wiki showed up. As in, you were supposed to learn about cave-ins when they kept happening to you when you build hallways too wide. After losing to cave-ins or floods a few times, you learn how to deal with them.
Because of this second version, I tend to prefer "Learning is Fun" to "Losing is Fun". Because much of the fun of the game is in exploring its little realisms and coming to understand how its mechanisms work through interaction. If you don't learn to understand how to respect water pressure and treat it with caution and understanding by taking the time to read up on the wiki... prepare to Learn the Fun way.
And as a corollary, losing that way is Fun only when you understand what you did wrong. Find out about the HFS the hard way, but learn to respect the cotton candy in the process? You probably have fun. Lose instantly because of a corrupt worldgen that dumps your dwarves in the magma as soon as you embark after spending an hour looking for the right embark spot and searching for a world with a rich history you'd enjoy? Not Fun at all.
Things that are completely random and beyond your control that just shit on your day is what makes real life unfun. Things that you can learn to predict and defend against, even when there are serious random elements involved that keep them from being perfectly predictable are Fun, however.