If you sit in the right seat of a saab/dash/whatever for 4 years you learn the same command lessons as going via GA. You just get to do it with a coffee top of climb. Been there, doing it, and if you are paying close attention you'll come out ready for command, and you'll know thats the case when the cockpit gradient suddenly goes from right to left when the captain has lost SA or is having a bad day and without a word being said the FO is making the calls.
When this stuff happens the FO will realise the cadet scheme they've been through works and its time to start looking for command. IMO its about time some changes were made so professional FO's can make the step up without going backwards first.
WRT rex c&t resisting icus change, thats standard rex empire building mentality; it'll be a seen as a reduction of their "power" to allow other capts to do their job, and they wont like it. I do think though that c&t need to approve which cadets can start on icus because not all will be ready at a set number of hours due different ability levels.
(formerly a cadet not with rex but in their system so aware of the issues and in the same boat as cadets wrt command upgrade)