I think Gibson's job at the Tahs lives and dies in 2018 on whether the team can improve their defence. It has been the key problem in 2017.
Maybe given the coaching options available and the budget constraints he deemed Cron a good hire for the forwards in an area that isn't his bread and butter and decided that he would take greater responsibility on the team's defence.
Ideally you'd have the budget to hire the best coaches in every area possible but clearly that isn't possible.
I don't mind Gibson putting his destiny in his own hands in 2018.
The overarching question is surely whether that 'putting (defence) destiny in his own hands' is a sensible, well-considered decision in relation to the resulting odds of team failure, or not.
Surely it is way beyond being about experiments re Gibson's talents or otherwise, but rather about the base viability of the Tahs as a team.
If the Tahs' 2017 financial position is so perilous that they have to cut back on elite coaching resources then there is only one way out of that rather serious problem and that is - play extremely well in 2018 and get into the 2018 S1X finals leading to better crowds, better sponsorships and finals home games to boost coffers.
If 2017's performance is repeated next year, the commercial and financial consequences will not be good I would surmise.
IIRC Gibson has no coaching background in defence and undeniably defence is any increasingly specialised coaching role that goes way beyond just the crude 'tackles made and missed' stats. It extends quite technically into ball-stripping from the tackle, high productivity aggressive defence (where the ball carrier is handled firmly backwards in a manner best positioned to set up the ruck and effect turnovers), holding up to gain maul start and then dominance, preventing offloads in the tackle, etc.
IMO, given all above, to expect a non-defence-specialist HC to also do that role - and when that role as you say is also critical to fixing chronic team-wide defensive lapses - to cull a defence coach position and place that task back on that HC seems like a super high-risk decision.