Usually when you come across a restaurant that still has an outdated sign, building design, or both, its in Podunk, USA, out in the middle of nowhere, in the boondocks, where the only people who would notice don't much care.
Burger King is usually the biggest offender in this area, in my experience. I've seen a BK in western Nebraska that was in a steel shed with the old 1970s logo proudly adorned across both the building and its sign. I've seen a BK in a truck stop outside of Des Moines that had covered the old logo with a giant sticker that had the new logo on it. And I've seen a BK in Pittsburgh with a Old West type treatment on its sign.
McDonald's and Wendy's have both kept the same, or very similar, logos and signs for such a long time that I've personally never seen one that had something different. And I'm the only one who cares about Hardee's, but I'll mention it anyway: their change in the mid-nineties from Brown-and-Orange to Brown-and-Blue and then their later change to the Carl's Jr. star logo caused much consternation on my part when the local Hardee's in Fort Dodge, Iowa where I grew up did not stay current.
If I was going to give out an award for "Fast Food Restaurants That Time Forgot", or Ffrttf's (pronounced Fifferretteff's), those three BK's would all be nominated. But they wouldn't win.