...Wouldn't most users end at first create various toolbox because they think it's a cool idea at first then end up always using the same (the one with all the tools) because they get used to it the most? ...
I thought about the exact same examples, and then I realize that sometimes photographers would use the paint tools too (to make small fixes here and there, to change selections in quick mask mode, etc.) and that many painters would still use other tools (selections, warp, transformations, etc.).
So would you really want to hide them (hence make it more difficult to get to them) even if to use it once every hour? At first this could look like a good idea but I really wonder if it will hold with the time.
You can already organize where things are by ordering tools in preferences > toolbox. Though clearly this is a good example of not very good UI. Basically many of the things we have which requires to go to the preferences are not very good in my opinion because I end up nearly never go there to customize (or forgetting you even could).