Here's my view. An airline needs Pilots, Engineers, Cabin Crew, and Customer Service/Check in staff. As a collective we represent the company and we are a tool to ensure the aims of our passengers are achieved. A reasonable amount of management and administration is required to manage us.
The rest are 'nice to haves' and when the chips are down should be the first to be reviewed. Look after your front line staff - they look after your customers, nee income.
I think that is a little naive.
Distribution - massive operation
IT - backbone of operation
Website - do you want a website like Ryanair? Thought not.
Cargo - a not inconsequential operation in itself...top 10 world carrier
Engineering - goes without saying
Finance - who pays you, accounts for the revenues
Carbon trading - legal requirement
Alliances - essential for BA in directing traffic over network
Legal - quite necessary considering global nature of business
Audit - safety, financial, IT
Yes, it is customer-centric, but have you for one moment considered how the whole network operation fits together? It is not, and cannot be Ryanair - regardless of the number of unnecessary managers, there are many more essential but non-front line staff around.