Does every shield need earth ground?
No. Some shields can reduce coupling through conductive continuity without a direct earth connection.
Why can a long ground wire be ineffective at high frequency?
Inductance can make a long wire a high-impedance path.
What should designers focus on?
The complete current and field path, including bonding, seams, cable entries and safety requirements.
Project note: Use supplier data as a starting point. Validate the finished assembly at the relevant frequencies and environmental conditions.