Yes, hullo, your point is valid but as we have seen in the past, 3kg "boom" apprentices certainly are worth following while they are "hot."
Due to their ongoing success they certainly get onto better horses but invariably soon lose there weight allowance or have it diminished and only the elite or select few go on to greater heights as senior riders(eg. D. Gauci, D Oliver etc). No offence to these riders but only those of particular talent prove to be stars of the future.
I'm mainly talking about those 3kg boys put on to relieve the impost of a well performed runner which is in the market for today's race but the connections of these horses in most cases would be better off utilising a competent senior rider due to the fact that less "dead weight" is carried by the horse.
Better balance, race tactics and experience displayed by the senior rider will in most cases outway the perceived advantage of lightening the weight the runner has to carry.

Cheers.