I've seen you do this a bunch of times and people explain it to you many times. Most players we know have zero interest in sitting around town doing nothing. They want to play the game and go on adventures.
Assuming that players want to play the game and actively do heroic stuff (whether given XP or milestone leveling) is a near-universal expectation to the extent that your assumptions on this particular area of player motivation are clearly more the exception than the rule.
I personally like XP, but I didn't change my behavior in a friend's Rime of the Frostmaiden campaign just because he was running milestone leveling, and as far as I could observe, neither did anyone else. To some extent milestone leveling could be argued to better support verisimilitude, because the group focused on our objectives and character interests, and weren't motived by trying to take on extraneous encounters or clear out rooms just because they'd be worth more xp.
Hypothetical player: "I gain a level after one session, right? So what if I say my PC just stays back in town while everyone else goes to check out the goblin raids?"
DM: "Then that guy never becomes an adventurer. He was called but found his courage lacking, and settles down in town with a safer job. Want to roll up a new character, or would you prefer to figure a motivation for this one to actually be part of the game?"
I want my character to survive and prosper. Prosperity might require adventuring, sure; but survival doesn't - and if I can gain levels by sitting around on the farm then it's clearly in my better interest (both as player and character) to do exactly that.
Again, as noted about re Curse of Strahd, Rime of the Frostmaiden is a hard-line adventure path with a clear sequence of events that are intended to occur (even if not necessarily in an exact sequence) and to which milestone levelling can be fairly easily mapped if not already noted right in the module. Further, if the campaign is set up as "We're playing this adventure path" then all have tacitly agreed to doing so when signing up for the game.
But - and I keep having to say this - not all campaigns are single adventure paths, which means when signing on for the game all you're committing to is playing your character and showing up for the games. There's no other inherent restrictions or expectations other than those you bring yourself.
And 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. Solution: close the damn loophole and use a character-action-based reward system, preferably individual-character rather than group-based.
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.
Hypothetical* player: "I gain a level after one session, right; and so does everyone else? So guys, why don't we all just stay in town for tonight's session, level up, and we'll be better able to take on the goblins next session?"
DM: >
smoke rises from ears <
* - or maybe not so hypothetical, as I could easily see myself saying exactly this.
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.
Downtime is part of the game, no matter what you insist the "vast majority" of gamers want. And even if you're right about popularity, who the heck cares?
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!
Player: ok, how about I come with the group, but stay in the back and contribute as little as possible to the party's actions?"
DM: " Then you're fine. Welcome to level 2, hero!".
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.