I'm not insisting that it's "not possible" to run a fun level 14 campaign. I'm saying it has a different format, and in particular, I'm suggesting the OP's request for "sidequests" is difficult to fulfil.
Whether of not a high level game is fun or not depends on what you like.
I'm saying the format of some of my (very popular) high level campaigns is of the kind you say is not possible.
An RPG session 'side quest' can involve major opposition, but it's not the main point of the campaign and it's usually short. Look at eg the high level Delves in 4e's Dungeon Delve.