Should the typical campaign change at high levels to take on a different tone?
Tone is something I really don't see as relevant to level - unless one sees increasing prowess in class abilities as tone defining.
For me, tone is completely directed by the players engagement with the game. It is in part created by the adventure, but I see my role with mood and tone as a DM as largely mirroring the players' emotions. If they're scared, I don't detract from that in my presentation. If they are taking the game comically, I present it as such too. The material remains the same, but the mood I present changes. (Interacting with NPCs is different though as they have their moods built in)
I think the question really is, "
Should the underlying theme of the game evolve according to a character's or the group's level?"
My game distinguishes between material one engages in at low level and at high level. It also allows 1st level PCs to adventure with high, say 10th level, ones. Ultimately however, it all gets mashed together by the players.
For me, class level determines different class challenges. Each level of each class has its own basic competencies to be mastered by a player. But there is a lot more to a character than their class, which is the only part that does gain levels for me.
Wealth is not tied to class or level. Neither is one's standing in the monster communities of the game world (you can play a prince or pauper). Knowledge or information is its own reward as well.
The tricky part is, all of these are in part tied to classes. Some classes are restricted in wealth and standing specifically as a hindrance. Others gain these as a bonus at different levels, primarily the high level strongholds, but followers count every bit as much.
What I do is enable each player to determine his or her objectives and let
not gaining XP, and therefore class levels, be one of those. If they want treasure, they can focus on that. If they want combat, ditto. They can buy a peasant's home at game start, if that's the stronghold they want to build and defend thereafter. Ditto on gathering henchmen and hiring hirelings. If they go in for heavy trade, they can corner the market game. If they engage in intrigue, they and use diplomacy to gain political power and authority. If they build on that starting hovel, they are playing the civilization game.
Gaining levels is only part of the game as I see it, but it is the part the characters are best equipped to do too. Combat, magic, clericism, and thievery are not small parts. Keeping them intertwined in an adventure module is probably the more difficult part, but that design happens prior to play. If theme really changes according to class level, than its the scope of complexity rather than the variety for my game.