Between the anime's Endless Eight arc in the second season and the years long delay between new novels, Kyoto Animation (Haruhi's original animation studio) and Nagaru Tanigawa (Haruhi's author) pretty effectively and prematurely killed the series, albeit not intentionally. Or at least I think it wasn't intentional. Simply because you'd have to try really hard to fuck up as badly as KyoAni did in the second season.
Interest picked back up with the Disappearance movie which was welcome but, to the anime's fans, it was a fitting end. There was no saving the series with Nagato Yuki-chan. It's good and all, but it has a different studio, different plot, different artstyle, etc. To those unfamiliar with Nagato Yuki-chan, I suspect it's simply off-putting. To them it isn't Haruhi. And in a way, they're right, since it's written by someone else, anyway.
It's not necessarily boredom, but the lows were very low and harmed the franchise. Realistically, the series may continue on. but the movie was probably the last high point.