The thing is, the whole topic is not fully binary. I agree with you that having a rootable phone does increase security in certain ways.

> What doesn't increase security is when bank apps that are essential to daily life start detecting that a device has been rooted and force a lot of people into using closed source extensions to hide the fact that the devices is rooted.

I'll ask naively: Why not? I can come up with a bunch of arguments why it does help the bank and why it might reduce the risk of certain attacks.