In reply to both above posts, the issue isn’t even the boss. Or really the upgrades he unlocks. Sure someone else can do the boss and your grade school age kid can be replanting your carrots and feeding the pigs.
But then you kill the third boss. The adults are all out looking for mountain zones, and the kid is still at the “safe” Meadows base. One of two invasions show up …
Mountain invasion which includes flying monsters, so even if you put a wall around your garden they will come in. Or troll invasion, with sticks. Size of invasion and hit points of mobs scales to the number of player on.
So, one builder vs a Mountain zone multiplayer sized invasion scaled to the adults out far away. Not even going to be close. Them logging out will be best at that point. But there may still be significant damage. Later invasions are designed to be fought. And yes, swamp, mountain and even plains mobs will be coming into the Meadows with the singular goal of seeing how much stuff they can wreck in 2-3 minutes.
The bigger invasions are triggered by killing the bosses. So if you never kill the bosses on a designated “builder” save, it never has them.
That is why.
You can unlock copper (and everything) on a builder save. Have access to another save that is killing bosses. If your builder is a competent adventurer, have someone bring them an antler from the first boss. No issue getting more as you can kill them over and over. Once they pick up the item the game will unlock all relevant recipes for their character. So once they pick up the antler they will know how to make the pickaxe to mine.
If they are not an adventurer and perhaps build in cheat mode: Bring them one item of each new type. So for bronze age, bring them (copper ore, copper ingot, tin ore, tin ingot, bronze ingot, fine wood and a bronze nail). Have them pick up each item. And again, all recipes unlocked for them. Repeat with iron, silver, etc… You probably for this type of builder want them to even pick up items from the starting age like coal and core wood.
So raid events are tied to a server’s current boss state. Raid strength is tied to the current number of payers logged in. And recipes available and building parts unlocked is tied to the player’s previous unlocks and not related to the current save’s boss status. And, getting unlocks only requires someone brings you materials, not that you kill the boss.