Where is the reason no longer applying here? The Sages didn’t speak about half an hour, but rather about a boundary that changes according to reality, no?

Especially in light of the view of Maimonides and the Shulchan Arukh, that the measure is not fixed but changes according to “how much time remains until foot traffic ceases from the marketplace.” In any case, the other side of such an argument would be to say that someone who lights inside his home has no fixed time requirement, and this is also what Maharsh Austreich wrote. But in practice, most accept that there is a fixed measure of half an hour.

They did in fact speak about a duration, at least in light of the amount of oil one must put in the lamp from the outset (21b: “for its measure”). But even if they had not spoken about a duration, it is clear that when they said “until foot traffic ceases,” they took into account what that meant in their time. So everything I wrote still stands.

And this is something that needs to be factored in every time we talk about changes in Jewish law: there is the narrow perspective of the definition that was stated and how to apply it today (and therefore one would have had to require lighting until our own marketplace empties), and there is the broader perspective that takes into account the whole picture and the way of life in their time (and therefore there is no basis to create a new obligation to light beyond the time practiced in their day).
Apply that reasoning to other changes in Jewish law as well.