This is just hypothetical, but imagine the other 10% consists of die-hard nutters for whom it isn't just about the money?
Then there are two branches.
One has 90% of the hashpower and has blocks every 11 minutes.
The other has 10% of the hashpower and has blocks every 100 minutes.
The slow chain has only 10% of the transaction processing capacity that it had before the fork. 1 MB every 100 minutes. It quickly becomes unusable.
Checkmate.