I mean, when Eberron came out, it was touted as a setting designed for modern (WotC) era D&D, which at the time meant magic shops and any race being any class. But I think if Eberron has just been that, it wouldn't have lasted. Eberron brought a lot of unique flavor to D&D you didn't get in Greyhawk or Realms, and that flavor carried it into 4e and 5e. So yes, a 5.5 designed setting would be preferable, but it needs to be more.
Thing of it this way: what unique things did Nerath/Nentir Vale bring to the table besides perfect compatibility to 4e? What was it doing that couldn't be done in Greyhawk or the Realms? (And that's not a slight at it, I think it could be a great setting for 5e, but it needs a hook better than "the default setting for 4e".)