Is it expected that a junior employee will remain as such for 5-10 years? Of course not. Junior employees are promoted and given more responsibility, or they are fired quickly if they are not performing well.

Do companies let junior employees work on whatever they want? Manage their time as they want? No, one of the key indicators of an employee status is that their time and work is managed by their employer.

If a junior employee works on some passion project related to their work but on their own time, do they own the intellectual property of that work? Probably not; many employers demand that all IP created by the junior employee belongs to the company.

I think this is a case of “be careful what you wish for”.