A merchant will always be a sink to dump in the valuables you find while exploring or digging. He is a good place to introduce items that improve QoL, and are not game changers in any biome. Some such items can be locked behind bosses' progression but shouldn't be required to play the game, even in a competitive/coop way. Now what when there are no consumable items and no items that can be needed in several places?

In Valheim, Haldor could sell some consumables that are normally craftable by the player but are locked behind a boss/a biome. These items wouldn't unlock their recipe upon buying them, and they could be used as if the player had crafted them.
In short, a (very) rich player could bypass some game's linear progress, but not on a definitive basis, only on a item by item basis, if there is enough money, that is if player's main activity is exploration or to a lesser extend mobs' farming. Let's recall that maxing out exploration is not required to "win" the game nor to renew the daily consumable, thanks to breeding; growing and mobs respawning.