IMO it's kind of making a fuss out of nothing because this isn't a "loose end." Tenryu, or the Celestial Dragon, is basically touhou's version of the Big Dipper, and it wants to devour the north star to gain control of the heavens. WaHH isn't the first time it comes up in the series (that'd be CoLA ch 22, which isn't one of Rinnosuke's usual nonsense theories since it's backed up by WaHH 25), and I think it's nothing more than world-building information as the timeline for it happening is really, really far in the future (thousands of years at the least). Even when it does happen it's somewhat outside of the scope of the series itself - the fate of the stars is something you'd find in Gurren Lagann. While it's vague as to what will happen when the north star is eaten, and that can be speculated on to kingdom come, I feel that it's an event completely outside of the scope of the series.
On the note of speculation...man, ZUN has so many great characters to draw back on for season based shenanigans. It's a tragedy that we'll see exactly none of them since even when it makes the utmost sense ZUN practically never brings back old characters for the mainline shuute games (ignoring Alice and Yuuka, we've gotten Cirno like 3 times, Yuyuko, Kogasa, and...uh, I think that's it?).