Kaiten had one big issue. You had to use it 100% of the time on your Iaijutsu or the potency would be a waste.

The animation of Kaiten was really appreciated by the community, and the effect of x1.5 Potency on the next attack was also extremely good.

On the last iteration of the Samourai, you had a huge difference between Minimum damage of a Midare (No kaiten, no crit, no DH) and Maximum damage (Kaiten, CRIT DH). (Of course this is all an example, nobody should use Midare Setsugekka without Kaiten first).

But, even if people tend to love that style of Gameplay, I think a Major issue did rise up to the dev team. Kaiten itself and the rigidity it brings to the class (Not doing damage if you don't use it correctly).

Why ?

Cause Kaiten is actually no different from Dark Arts in the principle (Press a button to upgrade next attack). The dev's team seems to have reached a wall on the evolution of the Samourai for the future patch and I guess that Kaiten was in the way of this evolution. Getting rid of Kaiten may allow them to modify what Samurai will experiment later on OR modify how you use your Kenki gauge.

If you think about it for just a second... Don't you think that managing the Kenki with various ability (Not only Shinten/Kaiten) can add a great depth to the class ?

I think that now that Kaiten is gone, they have more flexibility for the future.

But, should have they retired it like how they did ? No. Absolutely not. I agree with the community that it was a bad idea... If they had removed it in 7.0, along some rework, I don't think people would complain like that.