Again: if you make a thread about Sentai endings, people are going to argue about Jetman in it. I'm not going to mod anyone for talking about Jetman in this thread.
I just wanted to point out that I don't think it's accurate to characterize the Jetman novels as "fan service" since it's not known whether Inoue wrote them before or after the series, regardless of when they were actually published.
It's really,
really unlikely that novels published after the series were written before the series was made. It's also really,
really unlikely that more than, say, one chapter and a treatment describing the rest of the series was written before the publisher had secured the licensing agreement and finished out a commission contract.
Commercial writers generally don't write anything of substantial length before they've gotten their agent to find a publisher, to avoid wasting time on work they can't sell. Inoue was active in television writing for about ten years prior to Jetman, and probably would've been kept plenty busy churning out pitches and episode scripts.
Finally, the particulars of the Jetman novels sound
way too much like typical fanservice novel material, down to the trilogy format and the content, for me to find it terribly likely they weren't written specifically to be that sort of product. If Inoue had written them in his spare time, then
surely you'd be describing something less generic to me.
I kinda wonder if that was originally supposed to be a lead in for a post-series straight to DVD thing like Shinkenger and Goseiger got...
... And then they decided to do Super Hero War instead and... yeah.
I don't think we were ever really meant to see the Gokaigers fight the remaining Zangyack. The outcome of that battle isn't in doubt in any way, it's clearly just mopping up remains. The point of Gokaiger's ending is that the team goes from expecting a magic cure-all to solve their problems, to being willing to solve their problems through their own effort.
If we did get a Gokaiger Returns sort of thing, it probably would've just been character fanservice with maybe a couple of cameos. I'm not sure Super Hero Taisen would be the reason why we didn't, since that was a totally different production team. I wonder if Toei just decided that the format wasn't sustainable for shows that weren't Shinkenger.