Just a little correction, if all witnesses stop to post units, user units will be validated by other users posting units. Users help each other.
In plus of validating users units as normal users, witnesses help users as milestones/lighthouses to find the path until the genesis unit. Once the witnesses' majority crossed, the last witness crossed acts like a shortcut until the genesis unit.
If witnesses stop posting, users will take a lot of time to reach the genesis unit... or the last witnesses' majority and so a shortcut to the genesis unit. The genesis unit could be really far away.
RE: Witnesses in Byteball