But in all seriousness it isn't just Sentai, Kamen Rider and Ultraman have been declining ratings wise for the past decade (inb4 blaming Decade).
In other news, after Build Fighters aired /m/ did a giant thank you picture where people posed their gunpla resembling Sei's victory pose in episode 24. If the pictures are correct with the no duplicates 309 people in total contributed to the collage. It was supposedly sent to the official GBF twitter account, but everything after that is a mystery. Said picture can be viewed here with a proper image hosting link.