"plain and dated-looking"

Full on retro would be an interesting concept. Reusing box art from 1960, assuming its still in a vault somewhere, is cheaper and faster than trying to upgrade 2005 box art to 2016 current fads. And it would be kinda cool looking.

Two problems: Stealth inflation by shrinking package means the "one pound style" package will now be huge next to the current "one pound style" packages that are down to only 11 or so oz now. I suppose scaling in photoshop is pretty well understood. The other problem is the sports athlete on your 1961 Wheaties cereal box might have turned into a serial (cereal?) killer sometime in the last half century, need to research and censor some famous personalities.

I think the 'value' house brand's plainness also helps to nudge less price-conscious customers up to the next level up of own-brand stuff. Tesco used to have three levels of own-brand: the blue-and-white-stripe "Tesco value", a more "normal" looking Tesco own-brand, and the premium "Tesco Finest". I suspect the dated plain look of the 'value' range helped it avoid cannibalising sales of the mid-range versions of the same items.