The primary reason why I believe that it is imperative to have a diverse background is that a pilot moves up
gradually to increasingly complex tasks and increasingly complex systems and becomes
gradually more at ease at flight management tasks that
all pilots must accomplish on every flight
And why it is especially good to become a flight instructor/GA commercial pilot for at least 1500 hrs before moving to airlines is because the monitoring ability of a flight instructor is extremely high. this helps later with CRM ---monitoring the other pilot while simultaneously managing with traffic lookout, weather, communications etc. and all the while flying with pilots of different abilities and knowledge levels.
Flying simpler airplanes for a while as a GA commercial pilot is also excellent as you learn to deal with passengers and accept responsibility as a PIC then when moving to airline ops perhaps the candidate's basic skills [instrument scan] will be much easier to refresh and their ability to participate in the CRM process is greatly enhanced
further more there might be a greater acceptance to having to revert to basics.
Jets are just another type of nice airplane
if you read carefully yes, I am on topic!