I think you have the right of it. From what I've read on this, it sounds like Kaiten was difficult for them to design around, but there isn't a lot of detail on why exactly. If you look at these changes through that lens, it makes sense that a primary goal of these changes was to remove Kaiten. I suspect that the devs locked in on removing Kaiten early in the process and that explains why they did not do some of the button merging ideas players have suggested as alternatives that would be more aimed at reducing button bloat. Essentially, button bloat was not the problem the devs were trying to solve.
What I'm unclear on is what the nature of their "restriction" with Kaiten was. Was it just causing problems for patch to patch balancing or was more of a difficulty when trying to make changes for major expansions (like adding new actions). If it was the latter, I 100% agree that they should have waited until 7.0 to remove Kaiten.
Ultimately, my biggest problem with the changes is that the rotation feels hollow now. Without something else to spend Kenki on we're left just spamming Shinten and Kenki may as well not even exist anymore, and that just isn't very fun. I think I'd be more ok with the changes if they added something else to spend Kenki on that would impact the rotation in a meaningful way.
A translated quote from Yoshi P. from the FFXIV Reddit Discord Translation\Live Letter Channel (
https://discord.gg/ffxiv):
"...and on Kaiten getting removed we did that because we want to do away having to be restricted by that move"