Tori better start thinking about closing these movies, b cause the box office is only gonna get worse.
They do better than the Movie Wars features at least. The box office of Rider movies in general has fallen in the last 3 years, but that's not only the SHT movies, but all three - solo movies, MWs and SHT. Even so, SHT still does better than the MWs and solo movies for the same current show (Aside from SHTZ, which did worse than Ultimatum and Wizard's solo movie, IIRC).
Shirakura was also the head producer responsible for Agito (the most successful Heisei show), Ryuki and Kabuto (one of the trilogy of fail) though. I could see Yonemura being responsible (he is not a good writer), but how much is Shirakura actually responsible as a producer other than publicity given he has more important duties such as being part of Toei's Board of Directors.
Shirakura is responsible, at least publicly, for the basic concept of the movies and sometimes their gimmicks. So, elements like these movies always being hero vs hero is due to Shirakura.
However, at this point, I'm not sure these movies would even exist without Shirakura. Look at the Movie Wars. The producers there seem often satisfied in going with two shows, even though it's been shown that they can go with multiple at once when they feel like it (like in Megamax, Ultimatum and Sengoku Battle).
Is the a reason Toei still keeps him on, then? Or is there stuff he's written that's actually good, but the talent just doesn't carry over to his toku work for some reason?
It could just be preference. I think these movies certainly could do better if better handled, but they aren't doing particularly bad either. However, there are also other factors. It's not every Toei writer who's willing to handle characters from multiple shows at once. Also, these movies often seen to have production limitations and directives weighting a lot in the writing, and yet again you won't see every writer being able to change and tweak their script according to that. For example, the original SHT's script had to be heavily changed once they actually confirmed Kaito's actor presence in the movie, and he's pretty central to the final script.
Although I haven't seen clear comments about production changes for the new movies, it's still possible to see things like how Shotaro's actor was never on screen with Kouta's or Takumi's actors in KRT - or how Fujioka is actually in only two scenes in KRT. So, the script clearly had to be written or changed based on the actors' various schedules.