This is gonna be a long one folks, so grab some tea or something lol
As angry as I am about Kanji not performing at the Budokan, It's impossible for me to see this as a 100% management decision this time, rather than the girls influence. I know there are girls like Uemura, Yuipon and Zuumin who have voiced their want to change kanji so it's not "techi and friends" and have confidence in performing without her, and there are a lot of members who have voiced their wish to perform with everyone. Since the Budokan is a special venue the girls may have wanted to do it as 21 or not at all. It's only a theory, but it seems likely given the attitude of the group. Let's remember that Techi has missed a performance before last year, and Kanji were only performing for one day at the Budokan, so this would be no different to that time except this time, they can prepare for it. That's why I find it hard to believe that techi's absence will make management completely pull Kanji out of the 3-day live. On the other hand, I recall the last concert (I think it was Zenkoku?) which was prepared 1-2 months prior. The concert was very theatrical, and was based as a story with techi as the center of that story (also confirming that the singles were all linked together, being built from Saimajo, focusing on techi's story until Kaze ni, where they hint a supposedly new center for the next single). If techi wasn't present, it would be very difficult to pull off..pretty much impossible. If this concert was anything similar to that, it also would have been very difficult to continue without her. A very weird analogy, but i sometimes compare Keyaki to Final Fantasy. Final Fantasy is a franchise that has 15 titles, all holding different stories and sub-stories. FFI's story is different to FFII, FFIII as so on. Keyaki is the same, we're focused on Techi's story right now, in the "next chapter" we might focus on Neru's or Zuumin's story which has been built on since debut, or Uemura or Yuipons stories that are so unknown it puts all cards on the table in terms of keyaki's style change.
At the same time, a lot has to be said about the way they have promoted the group thus far. Rotating every member's position for 5 consecutive singles except Techi only widened the gap between her and the group. The only way to possibly fix that "gap" would be to change center. I can compare it to Nogi where during the first 5 singles, we had Ikoma at center, but their fukujin was quite solid as well, so when the center changed it wasn't a huge deal. Up until now, even on the performances Techi is absent from, they never "replace" her position, so it's like an empty void or a double center by whoever was beside or behind her, never a "official" center. The groups motto of "humility, kindness, bonds" has made them take an approach of "no one can be replaced" as seen when they didn't sub Zuumin. In a way, both the groups ambition to stay in a 21-formation, and managements poor handling of these situations, has caused this problem that seems almost in-repairable. If they get a new center and the song doesn't do well, they lose momentum. If they mix kanji and gana's, fans and members might get angry and sales will go down. If they have a double center, the essence of a center is destroyed. If they give Gana's their own A-side, it wont do well. There are so many disadvantages that it seems like mangement doesn't want to risk it, and when the group is constantly on the rise, why should they?
Lets be honest, Keyakizaka46 right now, is Kanji. Hiragana's have been put on the back burner since the group was formed, and only grew popular when Neru herself became popular. She became so popular that SHE became Hiragana, so they just moved her to Kanji where she always belonged. Hiragana members are amazing and have been given lots of cool things like a new Drama and features on Keyakitte, along with their own one man live which they did very well, but it's never been on the level of Kanji who have been in the spotlight since debut. If they're serious about being a separate group, giving them the opportunity to have a 3day live at such a prestigious venue is a great way to asses their popularity. If they can sell out in less than a month, and hold an amazing performance, I think management will be confident in finally pushing them to do an A-side (be it a double a-side, a senbatsu, or a completely different single) which would further push the group. This leaves me excited to see the performances despite the exclusion of Kanji.
I'm mostly worried about the members, Hiragana must be feeling the burden of performing 3 days on their own, knowing their history of being in the shadows depends on this concert, but also sorry for the Kanji's who have essentially "handed the budokan concerts to them on a plate". The Kanji members must feel frustrated and lost, maybe even useless or helpless that they can perform on their own. Techi especially will be feeling the brunt of it all as the report made it seem like the cancellation was because of her injury (bad wording from management tbh, 0/10 for effort). Physical injuries will heal, but it will effect her mentally. She's still only 16 years old.
From a business standpoint, I can see why the decision was made. As a fan, I can understand why fans don't like this idea. Personally, in all honestly I wasn't too thrilled to see them announce a budokan so shortly after the kouhaku incident, and I was worried for the girls and their health. Rehearsals for concerts can take a lot out of them, and we've seen how much the kanji's push themselves, so I feared they wouldn't properly give themselves the rest they needed. Essentially, they have the next two weeks off, and it gives Hiragana's a chance to shine and prove to us that they're on the same level as Kanji and should be treated as such. So i'm neither happy or sad. Excited? Maybe. Looking forward to what else the group throws at us this year lol 
I hope Kanji can be Techi's lightning rod, and I wish for Hiragana's to be Kanji's too, even if only for the 3 days.
But it made me see things from a slightly different perspective. Also, the singles make a story thing is quite interesting, but I can totally see it! Now I'm seriously hoping for a new center, because adding this Kaze ni hint to the story... well, it would be epic to me. 
Also can't wait to hear the songs. 
What's the process usually? I never experienced a Keyaki release myself. Is it similar to H!P like we get event rips and radio previews?
Congrats! 
(I was hoping for this since I saw that some other groups's Unders got their own album earlier.)
Hiragana-chans, to me, look like they're from AKB. 
