It has a different timing. That's the key difference. The text on Highway is unlimited scope because it is below the line, nothing to do with it saying "While this is in play". The line opens up the scope of the text to apply for the whole game. That text has "While this is in play", which limits the scope of the following text describing cost reduction. You (and I think this is more Awaclus than Jeebus) can't claim it as precedence for the global scope of the phrase; that is already fully explained by the dividing line. It works the same way with all other cards with dividing lines, e.g. reactions, Embargo, when-gain, when-trash.
If you want, you can say that
when you play this, { card text }
is in unlimited scope, because it's always true and you re-execute { card text } every time you play the card. But { card text } is within the scope, "when you play this".
Look, each card is like a little computer program with instructions that you resolve from top to bottom in order. You play a Shanty Town. It says "if (no cards in hand) { draw 2 cards }". That if-statement is limited to the scope, "when you play this". It doesn't exist forever. In a later turn, if you have no cards in hand, you do not get to draw 2 more cards.
So why should a while-statement be any different? Why does it have to exist forever?
I agree with everything you say here, so I don't really understand your point in using this to refute what I said about the difference between above and below the line. I don't think you actually read carefully what I wrote in my last post. I'm happy that you've come around to both the fact that above-line has "when you play this" in front, and that when you include that, both above-line and below-line is unlimited in scope. But I don't know if you agree that the only difference is that text. That text is what limits the scope of { card text }. I'll get back to this.
First, your interpretation of Squeegee is not very intuitive. The intuitive interpretation would be Haddock's. (I'm not saying that intuitive is necessarily correct, but that's the reason Squeegee was used as an example.) You're saying that Squeegee only works if you play Gold before you play Squeegee. And then Squeegee works as long as that Gold is in play. It makes it consistent with your interpretation of Altered Highway though, I'll give you that. You and Haddock can argue about Squeegee now.
Not only that, if Awaclus's interpretation of Bridge is correct, it is always sustaining that conditional check and it should never be discarded either! That's clearly not how Dominion works though, is it?
You can forget about all your arguments about not discarding, because it only applies to Durations. That's the reason Duration is a type with special rules. Possession, as AJD mentioned, is an example of a card that actually wouldn't be discarded if it was a Duration, but it isn't a Duration.
Now to the core, again. I like your computer program thing. I represented it kind of like that too, in a post where I did all the cards.
First of all, punctual effects like +$1 don't need a scope. It does one thing that per definition can't extend. It adds 1 to your pool of coins. (Of course there's a general rule saying that your pool of coins starts at zero at the beginning of the turn.)
Here's how I would represent some cards:
Smithy
Unlimited: When you play this, { +3 Cards }
Moat
Unlimited: When you play this, { +2 Cards }
Unlimited: When another player plays an Attack card, { Unlimited:* you may reveal this... etc }
Highway
Unlimited: When you play this, { +1 Card, +1 Action }
Unlimited: While this is in play, { Unlimited**: cards cost $1 less, but not less than $0. }
Hireling
Unlimited: When you play this, { Unlimited: at the start of your turn***, { +1 Card } }
Altered Highway
Unlimited: When you play this, { +1 Card, +1 Action. Unlimited: while this is in play, { Unlimited: cards cost $1 less, but not less than $0. } }
*Moat's "reveal" can actually be done several times in the defined timespace.
**The second "unlimited" on Highway means that there is no further scope defined within the outer scope ("while this is in play"). It's valid throughout that scope.
***Hireling clearly doesn't need to say "for the rest of the game": see Prince. It doesn't need to say "each of your turns" either: Not specifying a turn would mean all turns. Just like "while this is in play" means every time it happens. Yes, I know that one is above the line, and the other is above. But I'm trying to show that there is no difference except for the assumed "when you play this" above the line. All non-punctual effects are always unlimited, within the scope that is defined "outside" of them.
Please note that Altered Highway just takes the below-line effect from Highway and wraps it in "when you play this". Not adding or subtracting anything.
EDIT: I realized I'm wrong about Moat. What follows "when another player plays an Attack card" is not unlimited. If it were, you would be able to reveal the Moat and get that effect for the rest of the game, whenever you have it in your hand (same for Secret Chamber etc). The same applies to "when you play this". It's not like after you play a Smithy, you can just keep drawing three cards whenever you want. So there is a difference depending on the type of effect. This is a bit more complicated than I thought...