Well if it was a halfway decent replica, it would be made out of the same thin material, with the same 70s colors, the same large shorts, the same stripper boots, etc. Hence, "dated."
I really don't get fashion. I don't understand how something can look good in one decade yet look bad in a later one. Surely quality or beauty is timeless. Would anyone criticize a rainbow or a sunflower for using last year's colors? If something is deemed inferior just because it isn't current, that's got nothing to do with aesthetics or quality, merely with conformity.
That said, I decided to compare photos of
Reeve's costume and
Dean Cain's costume from
Lois & Clark. I don't see any material differences in design aside from the shape of the belt buckle and the shape of the boots. Maybe Cain's shorts are a tiny bit smaller, but not so you'd notice. As for the colors, the Cain costume's hues do appear to be richer, which I do consider nicer because I like richer colors, but that's got nothing to do with decades or fashions; I liked them just as much 30 years ago. Then again, one can't always trust the colors in photographs, particularly clips from old films. The processes used to transfer film onto video in the '70s and '80s tended to wash out the colors, so that audiences have grown up thinking those films had blander colors than they actually did. I know this is the case with
Star Trek: The Motion Picture, and therefore it stands to reason that the same would be true of its near-contemporary Superman films. So the apparent difference in colors should be taken with a grain of salt.
I will grant that the construction of the Reeve costume seems simpler, the materials and stitching and so forth. I suppose that might be called "dated" if it represents an actual improvement in technique and materials, rather than merely a difference in the degree of care or workmanship involved.
As for
Routh's costume, it looks to me as if his shorts are actually bigger than Reeve's (and no, not in
that sense, just in that the cut of the leg holes is lower). The boots are smaller, but I don't know or care enough about fashion to understand what, if anything, that has to do with the date on which the costume was designed. The belt buckle is replaced with an additional S shield, which merely seems redundant rather than "modern." And the colors are too subdued, maroon replacing red. I understand the value of using darker colors on the big screen, but this goes too far at the cost of aesthetics. The colors aren't richer, just duller. Well, the blue's not bad, but the maroon isn't good at all -- and if anything, seems even more "dated," like the kind of color they might've used in the '40s or '50s. As for the chest shield, its small size evokes the Superman comics and cartoons of the early '40s, and its faceted look suggests Art Deco more than modernity. (And it doesn't make a lot of sense. How does Clark hide a protruding, textured chest shield under his shirt? It's implausible enough with just the bright colors and the cape.) So I don't see how this represents a fundamentally more "modern" design than the Reeve costume, except perhaps in the construction techniques and materials.
On the subject of Spider-Man's costume, I wouldn't have minded seeing Peter get an intermediate costume with a "homemade" look but the same basic design, and then use the winnings from his wrestling career to get the fancy one made. But since they truncated the origin so that Uncle Ben's death came after his first and only wrestling bout, that wasn't really an option. I'm hoping the new film will do something like that, but I still don't like that rubbery costume they're using.