It all comes down to this distinction:

Broken bikes are broken indefinitely - coz the nature of broken bikes is that we don't know how serious it is.

broken bikes will alter the capacities of your categories and product lines.

Mechanic Reservations have a duration - coz it means that someone has assessed the bike and determined how long it will be out of action for.

(just like a reservation for a client - only its an 'in workshop' reservation)

when you're full or nearly full having broken bikes will cause you pain.

coz it takes out that bike going forward indefinitely.

so setting them to mechanic reservations will limit the impact.