PCs begin the game at level 1 in layer 1 of the dungeon where a parabolic distribution averaging level 1 for treasures, monsters, and traps exist. The PCs may go to harder levels, but they are choosing to do so. Harder levels can be recognized because they are of greater difficulty and complexity. If the PCs rise to, say, 3rd class level and adventure only on the 1st layer of the dungeon, that's their prerogative. It won't end in much treasure or XP, but it is the players' choice. To try and put it in your words, increases in PC power do not force players into facing increased opposition. Nor does low PC power force players into facing lower opposition. The game only begins at a level appropriate area as no knowledge of the world by the Players is yet known.
Setbacks mean a loss of resources and even PC death (i.e. starting the game over at 1st level). It is each individual's choice on if they wish to increase lower level allies', PC or NPC, power level by giving them resources. It is the group's prerogative on deciding how shallow or deep into the dungeon they will go, what situations they will face, and which ones run from all based upon their own read of the entire team's effectiveness. A mixed class level group in a non-predetermined, "level appropriate" world is the norm in a sandbox game, so this is a constant issue confronting the players. It takes critical thinking skills and real teamwork to increase in class level, especially when increasing one's own class level depends so heavily upon the degree of influence one's allies have.
Since the degree of influence a player has within the game has historically not been linked to class level a balanced influence across all situations for all players isn't possible. Nor is it preferable. It is a chimera anyways and a removal of player reward. A 1st level PC or NPC may be more influential than a 10th level PC given the right situation. This is before taking into account player ability. Knowing one's own assets and liabilities allows a player to judge what situations they want to face and what ones they don't. These judgments, or any player's preferences, feed into the decision making process of the player group about what to do.