I'd dispute that a $4 Highway is "strictly" better than a $4 average cost Peddler, because Peddler reverts to $8 value once in your deck (which has implications for trash for benefit) and because Peddler can cost a lot less. Also, TR/KC, etc...
Definitely a $4 Highway is probably better card than a Peddler in general, but the phrase "strictly better" should only be used when a card is strictly better in all circumstances.
Returning to topic though, I think the main problem with the $5 Highway is that there can be a lot of variation between when two players, playing the same silver/silver opening, can get their first Highway, and once you start getting Highways its much easier to get more of them. If you're playing Worker's Village / Highway or Ironworks / Highway, a lucky third or fourth turn Highway massively swings the game in your favour, whereas if you end up waiting till turn 5 or 6 you're at a big disadvantage.
On the other hand, if Highway cost $4, it'd still be an essential buy for that set, but both players would have the chance to buy one before the first reshuffle. So if Highway was $4 it'd make for less random games.
To counter that, of course, Highway probably WOULD be too cheap at $4. Even if its just a glorified peddler without +buys about, it'd still be too essential a buy in many sets at $4. Even now, its an essential buy in many sets at $5, but its just a card that makes for very luck-based early games.