Mariner needs to work on his defence and various things, but is it best for him to do that in the NRL or in Qcup where the stakes are lower.
I don't think Madge considers Shiba a better player than Mariner, I don't even think it's a kick up the ass for Mariner, I think it's a plan for the long run. If it's better for Mariner to fix up his issues in Qcup then that's where he should do it, and then come back into the NRL when he's ready.
I'd say that Shiba and Gos are both placeholders until Mariner and Piakura sort their games out and are ready for NRL week in week out... making sure they're in the best possible position to succeed before giving them the jersey.
I think both are signed to at least 2027 and both are in the broncos plans long term... does it matter if they spend the first half of the year (or however long it is) sorting out their game and earn the jersey.
Even when it comes to Piakura I feel like a trend that is picking up is running with 3 backrowers in the 17 and having them split minutes. Maybe 1 plays 80, but the other plays the first 30 (Gos) and then the bench forward plays out the game. Gos is then on the bench if anything else happens.
It's better if one of the 3 is a bit of a hybrid and able to play in the middle... which is where Riki has been for me... but Big Pete has suggested Piakura could pop up in the middle as well. Robinson has recently been making sure his forwards can play edge and middle, to give him flexibility across the squad.
Cows have been running 3 topline backrowers in the 17 for a few years now (Nanai, Finefeuiaki, Luki, Luciano and now Bateman) and having them split minutes.
It's a similar concept to your better hooker starting on the bench, but ends up playing longer minutes... you just take them out of the line of fire to start the game and then bring them on when they can impact the game... or if they can't play the full 80.