What about a requirement for a first-time 121 CA position to have a minimum requirement?
Perhaps in order to upgrade for the first time, the pilot must have 2000hrs in type. Once you upgrade for the first time, this limitation is removed.
I personally think this is a bad idea for a number of reasons. Remember, there are plenty of people with time in excess of 2000 hours who can barely raise and lower the landing gear correctly, so hours is just a number. Experience, competence, and quantity of flight time do not necessarily correlate directly with one another.
Technically, there are already minimum requirements to upgrade. Aside from passing the check ride, IOE, and a company line check, the new 121 captain must also be observed by the FAA if this is his/her first 121 command position.