To correct the record, Frostmaiden is largely a sandbox early while the party explores the setting and gains some levels, then gradually funnels into more of an adventure path later.
Your proposition that "
fi the system is going to leave such a wide-open loophole where I can game it so as to advance my character while doing nothing, I'd be an idiot not to take advantage of that" is bizarre. It's predicated on the assumption that the player is more interested in advancing their character power and ensuring their survival than on PLAYING THE GAME.
If you said this at my table the reply would be simple- "A session of adventuring. If you want to stay in town and take no risks, why did I waste my time prepping an adventure and why did you waste yours coming to my house? If you don't want to play the game, why did you agree to?"
If the group has agreed to play an adventure game and you come to my table attempting to subvert that goal and obstruct play, we'll have an out of character discussion about our shared goals. If they're not actually compatible, you'll not be a part of the game.
I don't know what point you were trying to make here. My statement about the "vast majority" of gamers was simply saying that people play D&D to PLAY D&D, not to say their character sits around town while everyone else adventures. Even open world/STRICT TIME RECORDS games like Gary talks about in the 1E DMG, where some characters may be left in town healing or training while others adventure, aren't games where players are just CHOOSING to have their characters sit out of the action because they'd rather not risk their characters. Gary's expectation was that players still want to play, so if a given character is FORCED into downtime (or chooses it for something like magical research at the COST of being out of play), the player would almost inevitably play using a secondary character or henchman so they didn't have to sit out of the fun!
Come on. You're a more experienced DM than that. You know that characters who try to hide in the back still take on a share of the risk, and if they act truly cowardly and don't contribute, the other characters will tend to kick them out or give them a lesser share of the rewards.
Same as in 1E. Monster XP is divided among everyone who participated in fighting the monster, no matter how much or how little. Treasure xp is divided based on how the PCs divide the treasure, and that's almost always equal shares, even if PCs take on varying levels of risk. The Fighter at the front tends to take on greater risks, but OTOH their player also tends to get to DO more.