I made a post about this actually.
https://forum.square-enix.com/ffxiv/...ZY.-Here-s-why
The tl;dr is that the devs are making changes to the jobs - of which SAM is but the first in a series according to what Yoshi P. has said - with the intent of making future iteration and innovation easier. Addressing SAM specifically, Kaiten was removed because it was "restricting" future skill/ability creation for as-yet-unreleased expansions. So, in order to make the lives of the job devs easier, the decision was made to just remove Kaiten. This isn't to say that job design and balance is an easy task, as it's not. But the underlying thought process behind it is one of laziness. It's a laziness that led to the current state of tanks and healers and it's now spreading to other jobs. The devs are no longer designing something with "fun to play" as the primary focus, but rather they are concerned about "upcoming issues that may or may not arise in the future."
This is a lazy way of doing things, as it's the easiest possible thing they could have done. Instead of altering Kaiten in some way that preserves it's role while simultaneously making it less restrictive, they chose to remove it outright. They could have done any number of things to reign in the supposed power that Kaiten has. The best option would be to abandon their ridiculous focus on two minute burst cycles. Of course SAM damage is going to be nuts in 6.0. It was bad enough that we got second charges on Meikyo and Kaeshi, but with every raid buff also leaning into a two minute burst cycle plus the insane damage of Ogi/Kaeshi Namikiri, it's only natural that SAM - as a heavily raid-buff-reliant job - would see bigger damage spikes. It's like they didn't think the 6.0 changes through and realize that SAM's would be dumping all their charges under buffs. If SAM was still locked to it's old 60s cycle from 5.X then the damage variance "problem" wouldn't exist and any alleged problems that Kaiten brought to the table would be drastically mitigated.