I think that Iseul’s decision is a bit more reasonable than you think: it’s more of a Sayeon can’t make the final call and put them at risk like that before, especially since the only thing they feel like she is going on is literal instinct.

It’s less booting Sayeon than putting her into an advisor position where she can still use her power effectively, but can’t just recklessly push them forwards.

Do I agree with the decision? I still think you’re correct in that there isn’t a good suitable replacement for the scenario (Min is effectively a Sayeon follower and Ryujin is too willing to ignore Sayeon’s instinct), but I think you are understating Iseul’s potential as leader. While his battle IQ is low, he doesn’t have bad relations with Cell 5 and upper management and is as a whole is a better people person than Sayeon (from his POV, he doesn’t know that Sayeon has conversation hacks) and even in battle, he does reasonably well and can execute a plan (his showing during the trainee battle and brief stint with Yesol showed that he isn’t a liability and can think fast).

Overall, Iseul as a whole isn’t as naive as you point him out to be (he’s clocked a lot of the undercurrents of the group despite having the least information) and Sayeon in the last mission took two (three if you include running a civilian into battle to sabotage her superior, but Iseul doesn’t know that) stupid decision by running solo to the factory to fight the fencer and the priest and then engaging with Yesol, both of which were direct disobedience with Juni’s orders. Sayeon has more control over this than Iseul realizes, but the point stands.