This isnt about assuming somebody has nefarious ends in mind, its about what data becomes available and what power you give somebody with that.
I am saying the data you are creating during usage will at some point become useable this way. As mentioned elsewhere in this thread, this only starts to work well once you allow for memorizing earlier conversations. Which means the product here is the chatbot + your personalized configuration.
Think about how your search pattern and internet usage is currently used to personalize advertisements and recommendations of stuff that is tailored for engagement. Which already went as far as influencing your decision to vote. This is a lot more powerful because it doesnt just categorize you into groups that got split-tested for reactions but allows for learning your values, shortcomings and decision making process. In essence you are explaining why you think what and what your weak points are. I find it hard to ignore the overlap to where the work on IBMs Watson led to a few years ago. Though in a positively framed manner with Juliane Gallinas "Patton in your pocket", a tool to aid peoples process. In the end its both operations in the information space that can be used to aid or exploit your thinking.
Brainwashing is a loaded term but thats what propaganda is. Its framing things in a way to get predictable reactions out of you and have your cognitive biases defend those reactions. We are talking a lot about miss- and dissinformation, but the missing context is a lot more powerful, especially when its used with cognitive biases that have you for example be very loyal to the first point of view you encountered and identified with on an issue.
What a journal bot is is a tool you use to help you with the shortcomings in your sense making and decision making process which is weighted for what you value. Giving that information away to a centralized entity is placing a lot of trust in that entity overcoming the economic incentives to use that data in an economically beneficial way. With the lines between economically beneficial and politically beneficial being very easy to cross.
In short, I find training a personalized crowbar for your brain and giving it away to whoever highly irresponsible.
edit: Framing it as brainwashing is an unfortunate but good example. It creates another reaction then using terms like influencing. You are likely to have an adversarial reaction due to you being smart and your brain not being up for any washing. Framing creates grooves people get caught up in with cognitive biases acting as inertia and gravity towards the grooves.