Centralized vs. Decentralized Intranets

The challenge I have to face currently, is to convince senior management (board level) that their own ideas are right. They are in fact: They want to harmonize their current intranets and replace these homegrown 10 year old solutions through a new state of the art platform.
Sounds simple: replace ten times operating and development costs through one, harmonize ten scattered editorial processes into one, gain efficiency, control, power, quality and security.

Maybe too simple: Nobody dares to really decide this issue. We are spending weeks and months in discussions, considerations, alternatives, business cases and evaluations.

There are tons of measurable advantages for intranets, ROI calculation is not an issue. But business cases for online media are never a matter of figures only – they are also a matter of belief. I never saw a business case that could not be torn into pieces using quite the same facts, but different assumptions.
How to measure efficiency in terms of fast and straight ahead publication, how to make sure that all relevant facts are considered, how to exclude known excuses?
That are just the common problems in creating business cases, but I feel that they are extremely annoying and especially their solution is becoming actually counter-productive if it deals with so obvious issues such as centralized vs. decentralized intranets?
Or can somebody please tell me something else?