As someone who plays SAM casually, I'm of two minds on the change.

On the one hand, playing SAM in synched content felt awful while Kaiten was around, particularly around level 60 -- you had a very strict Kenki economy where a full combo gave you just enough for one Kaiten, so it made it difficult to have enough to Kaiten both your DoT applications and Midare in quick succession, in addition to being able to use your gap-closer, unless you were in a fight that could reliably trigger Third Eye and accommodated sneaking in Enpi and Meditation as net damage bonuses. It would be difficult to resolve this without impacting the Kenki economy at later levels, especially when the job has multiple other spenders.
Logically, I also agree that it creates unnecessary button bloat to have an oGCD that exists only to precede Iaijutsu -- at that point it makes sense to just combine the two into one button, something I myself have suggested many times for other jobs, not just for bloat but also double-weaving windows. The argument that "knowing when to Kaiten was a mark of skill" could also be flipped on its head, that the rotation was designed such that only particular skills would warrant Kaiten, and that the illusion of being able to use it anywhere else made it a trap set for newer players. So long as it competed with filler spenders like Shinten and Kyuten, there was also the threat of fringe rotations cropping up that just dropped other spenders.

Of course, using low-level content as a justification for such a change is a poor excuse given how little content it benefits versus how much current content is also affected by the change (especially now that we can just spend Shinten as we get it when soloing, with the only limiter being pooling for 2-min buff windows), and "think of the new players" arguments are a slippery slope to making a no-skill game.

It seems popular enough of an ability looking at this thread (though I admit I don't know how many players were begging for Kaiten to go before 6.1, inevitably there were at least some), though I can agree with dev concerns about button bloat. Even though there are other buttons that could be combined (like Shinten and Kyuten, or the Shoha skills), the slots they would open would just be filled by other high level skills in 7.0, which just means they're more likely to be on the lookout for further skills to trim rather than to bring back...