Realistically the gag shouldn't be funny anymore, but somehow it still is.
Yeah, this is an exemplary “one-gag” comedy series—as you say, somehow the punchline keeps working long after what should be its sell-by date.
In another thread I had occasion to compare Hino-san no Baka with Sekai de Ichiban Oppai de Suki!, and what all three series have in common is the central premise of a notional “screen” (respectively: pranks/therapeutic boob-groping/“normal”) that prevents the MC’s from acknowledging that their relationship is (or could be) a straightforward yuri romance.
There’s a certain level of built-in reader frustration with stories like this, because real romantic “progress” means that the whole premise of the series goes away, but the good ones keep finding new ways to develop their themes and characters without rendering themselves obsolete.