MoonSong
Rules-lawyering drama queen but not a munchkin
Exactly. I think the big mistake that often gets made is assuming that "I don't want to deal with lots of fiddly character building stuff" is equivalent to "I don't care how effective my character is". They are different sentiments, and I think the former is a lot more common than the latter.
DnD is a game where character power is the primary determinant of a player's power over the story. And it seems to me that the players that tend to not care about fiddly build mechanics also tend to be the ones that care most about shaping the story. Handing them gimped prefab characters is just a recipe for frustration.
And if the prefabs aren't gimped (which would, frankly, be miraculous), and are well designed using the custom build rules, then you just ruin the fun of the optimizers. If the best characters come printed in the book, not many people have fun putting effort into coming up with creative ways to produce inferior characters.
The solution is that prefabs should be different from custom builds, but still similarly effective. Then, the storytellers can use the prefabs, and play effective characters, while the powergamers can have fun playing with the custom build system.
The problem with your suggestion is, not all people wanting high customization are powergamers, some of us rather do it to play exaclty the character we want or get as close as possible, and I personally rather play an underpowered character for the flavor and being unable to play that because the devs want to protect me from myslef is frustrating.
And I don't think good=perfect, if all pregen themes and backgrounds are good, there is still too much room for optimization (starting for the right combinations between the two), and because the designers cannot foresee all posibble permutations, specially since there always will be new stuff in the splatbooks. Please make the pregens good mechanically don't hold back the flavorfull cool stuff from customizers.