"You could argue it worked"
In the short term.
Look at the long term consequences. BT is now 12% owned by Deutsch Telekom and a complete take-over is a possibility. Ultimately the fixed line thinking dominated BT management's approach and they failed to see staying in mobile as a strategic necessity.
From O2's point of view it may have been welcome; the expression "Big BT" was a term of abuse around the Arlington.