I don't think Japan's declining birth rate is THAT big of a factor. Ultraman has been consistently outperforming Sentai for quite a few years now, and Kamen Rider is still a toy sales juggernaut (Gavv had the worst first quarter for a KR series since W, yet that first quarter still outperformed Boonboomger's entire year).

1) It's stuck in a terrible time slot (it's still competing directly against One Piece for a couple more months).
2) This is just conjecture, but I truly believe Sentai has been parodied so often (even by Toei themselves) that the actual series formula looks stale to the average audience.
3) Losing the Power Rangers connection overseas hurt this franchise a lot.

Wow you're correct. Sentai sales went from being half of KR to a fifth.

A new spinoff is in the works:

Formation Lap: Cleaner of the Galaxy!

https://lp.tokusatsu-fc.jp/share/articles/41592