No... I'm saying both cards would play out the exact same way. In both cases, it provides a time at which the ability stops.
Yeah, in both cases, it provides a time at which the cost reduction stops happening. It does not provide a time at which the ability stops existing. If you want to argue that Regular Highway's ability stops existing while it's not in play, then fine, all of Altered Highway's while-in-play abilities that have accumulated over time also stop existing while it's not in play, but just like Regular Highway's ability starts existing again when it enters play again, so will all of Altered Highway's abilities. The part that disappears while the card is not in play is just the part which follows "while this is in play", not the "while this is in play" itself.
This is very hard to follow for me. The only ability I'm talking about is the cost reduction. The "while this is in play" is a restriction on when the ability applies.
I'll try to explain it as clearly as I possibly can then.
A card with "while this is in play, X" does X continuously whenever it is in play. When it's not in play, it does not do X, but when it enters play again after leaving play, it does X again.
A card with "when you play this, Y" does Y whenever you play it. It does not undo Y when it leaves play. Whenever you play it, you get Y again in addition to the Ys that you've gotten previously from playing the card earlier. It does not matter what Y is, "when you play this, Y" always does the same thing for all different values of Y.
Smithy, for example, has a "when you play this, Y" ability where Y is +3 cards. You get +3 cards whenever you play it, it does not undo the +3 cards when it leaves play, and whenever you play it, you get +3 cards in addition to the +3 cards that you've gotten from it in the past.
In the case of Altered Highway, Y is "while this is in play, reduce costs". It does "while this is in play, reduce costs" whenever you play it. It does not undo "while this is in play, reduce costs" when it leaves play. Whenever you play it, you get "while this is in play, reduce costs" again in addition to the "while this is in play, reduce costs"s that you've gotten previously from playing the card earlier.
That Y is a "while this is in play, X" where X is "reduce costs". Therefore, whenever you play Altered Highway, you get a new "reduces costs continuously whenever it is in play. When it's not in play, it does not reduce costs, but when it enters play again after leaving play, it reduces costs again" in addition to those that you had from before, and if there are multiple, then costs will be reduced multiple times.