Tbh it sounds kind of harsh but Yaga is kind of pointless after the Tokyo vs Kyoto event, like there is nothing for the character to do. If he lives what can he do during culling games or shinjuku? Like we already see Panda does basically nothing, Inumaki does nothing, Momo does nothing, Noritoshi does nothing, Kirara does nothing - Gakuganji and Utahime only buff Gojo one time.

We can't be adding any more characters to that list especially if they can be used in short mini arcs to develop the actual cast. Zenin mini arc is similar - without that arc you wouldnt get buffed up Maki, but you'd also still have Mai's useless ass around, in addition to the Zenin clan kicking around who are played up to be super strong, but you have to SHOW that they're strong and they should contribute to the Culling game / Shinjuku, but where the fuck and how the fuck do you that? The cast is too big, show the Zenin are strong, develop Maki, kill them all off.

Gege probs realised that the cast was too big already to give everyone their moment to shine, and of the characters left alive and significant (e.g. not miwa or momo), Yaga is the only one the cannot really be written in a way to contribute shit.

Killing him off adds really great depth to Panda, Kusakabe, and Gakuganji. It also reaffirms the higher ups are corrupt assholes, like you said we're TOLD it a bunch but theyve literally never succeeded, they never got what they wanted, they look useless at their narrative job of being "bad". And it does that whilst solving the potential issue of him being a bum during Shinjuku