The whole point behind PPK conversions was how Shinten vs. Kaiten has always been managed. Kaiten provided the biggest boost to GCD damage you could get but it wasn't as much as what you'd get from your big 2 minute oGCD's. So you developed a priority system. If Guren/Senei are coming up, then you use them. If they aren't available, you Kaiten every Iaijutsu. If that isn't needed then you are free to use Shinten, unless you need to use Gyoten or Yaten in the near future at which point you need to make sure you reserve 10 Kenki for that.
Whether or not Shinten is better than Kaiten runs entirely on the relative PPK values between the two abilities. As of ShB and 6.0, Shinten was the Kenki dump while Kaiten had priority if Iaijutsu was coming up. You needed to use Shinten regularly though, to avoid Kenki waste, but you couldn't just spam it because you were always thinking ahead to the next Iaijutsu and needed to ensure you would have enough gas in the tank to buff it. Proper balancing of the Kaiten vs. Shinten potencies granted relevance to both abilities without neutering one or the other like we saw in SB, where Shinten was the preferred spender of Kenki (except for when Higanbana needed to be buffed, you were under raid buffs or Hagakure was on CD and you had 3 Sen).
One of the stated reasons for the removal of Kaiten was the "smooth" the SAM damage curve, lowering the peaks and bringing up the valleys. They could have accomplished this without removing anything simply by reducing the base potencies of Midare and Ogi while boosting the oGCD potencies of Senei and Shinten and maybe even our basic GCD attacks. They could keep Midare and Ogi decently high in terms of damage but not so high that they have such an outsized effect on overall SAM dps. They could compensate for any reduction in big hit damage by filling it in elsewhere, either through direct potency increases, boosting Kenki returned when using Hagakure, altering Kenki gains from our GCD's or something along those lines. Perhaps a "Kenki refund" of some sort, where proper usage of Kaiten while Meikyo is active makes the next Shinten free and deal 25% more damage. That creates kit synergy and incentive to put things in the right order without leaning on mega potency moves for too much total dps. And if that's too complex then you can simply trim the potencies on our big hits and boost the potency of the oGCD's and basic GCD combos like I mentioned above.
In short, there are far more elegant ways that this could have been addressed that wouldn't gut the job, but apparently throwing out some half-baked change for a problem that doesn't yet exist - much less needs solving - was the route the dev team decided to take.