Ahhhhhhhhhhhhhh what?!
Benji was a 5/8 in 2005 when they won the premiership. QLD had a fairly decent player by the name of Darren Lockyer playing in that position at that time, and he continued playing in that position until his last series in 2011. No way in hell a youngster takes that position from him. They have a choice, then, between a young, exciting actual halfback or a young, exciting centre-come-5/8 to play alongside Lockyer, and there's no way they choose the latter in 2005. Even with Thurston out, Benji's halfback partner Prince would have been preferred. Marshall may have got a bench spot, at best.
From 2006 until 2010, Marshall suffered severe injury issues. By the 2010-11 season, when Marshall was in career best form, QLD had 2 of the best in the world in their halves, plus Cronk established on the bench, where he has the size, defensive capability and overall skillset to fill in at hooker (something Marshall didn't have). In 2012, following Lockyers retirement, the Tigpies plank, Marshall loses form (particularly at halfback), and with that, all chance of getting elected to take their reigns for QLD. Meanings then sticks with his original plan of shifting JT to 5/8 and starting Cronk at 7.
In summary, no, Benji would not have been elected over JT at halfback, JT would still be the superstar he is today and Cronk would still be the best 7 in the world!