I may not have made the 'focus on what is core' part clear. The skills you prioritise should be for your core money making systems. If you allow your back office tech stack to dictate tech choices for your core products you'll have a company that can't compete or evolve.
Your back office is not unimportant and ppl shouldnt draw that conclusion simply because I say it shouldnt drive your tech choices. But efficiency gains should drive the cost and focus down over time for those support systems.
As for deciding what your core systems are, if the CIO and CTO can't decide that, then tech stack choices are the least of your challenges.