Agree with you with the necessity of diversification.
The issue is not Facebook issuing its own crypto but rather the potential control we let it take over.
I believe that the same reason related to control that an entity can people on people (combined with the new technology) helped to originate bitcoin.
The need to ensure no one ( including governments) deciding people's life by manipulating own belongings (by tax or rates manipulation).
That's a the same anarcho- libertarian logic approach here. Anyone should be able to decide whether or not putting your eggs in the same basket.