And while randomizing the existence of adamantine may have a similar effect, I doubt anybody would use it. Not just reducing your chances of getting adamantine in an embark, but actually having the chance of no adamantine anywhere, is a bit scary.
Yes, that's the point I'm making.
My point isn't that randomly not having HFS in the world is a great idea, my point is that
the actual effect of this suggestion is basically the same for a very large percentage of players. I'm saying the suggestion is a bad suggestion specifically because it's functionally the same as not getting adamantine anywhere for essentially all intents and purposes.
Especially for new players, the players that the whole concept of the spoilers is aimed at, they aren't going to be reusing worlds simply out of a desire to reuse worlds.
The ultimate result of HFS rarity can simply be boiled down to a percentage chance of simply not having any in your game. Yes, you can reuse worlds, and hypothetically find some the next time, but what you're suggesting is that a majority of the
newbie playerbase is going to go building a full-fledged fort, dig down to the bottom, not find a spoiler they hypothetically don't even know exists, and decide to retire there, so they can go embark somewhere else to have another blind random chance to see the spoilers? That's not going to happen.
The only people it could possibly appeal to are the extremely slim minority of veteran players that are very well aware of the HFS and its dangers who are actually playing dozens of full-fledged fortresses without actually committing to any one, but want to have one fortress out of dozens to be "special" by being the one that has access to the bluemetal out of all of them... and again, that's still something that can be accomplished by DF Hack simply having some random percentage chance that changevein be run at the embark level as you embark, making the change on a per-embark basis.
Even with all that, however, you're still running headlong into the fact that even that extremely niche desire will become obsolete next update when Toady starts adding in cosmic bridges between different dimensions besides just the HFS, making fortresses have access to different places and metals, anyway, without the "you either have an end game or you don't" nonsense.
The other HFS options like spire size are, as I previously mentioned, not desirable either, because they just
beg for players to game them for maximum exploitation at minimum risk.