In this case, while the effect is not immediately catastrophic, they are looking at many months before the problem is fixed and they have caught up with the backlog. In the meantime cars due their first MOT cannot be either MOTed or have their MOT extended (nor can taxis), as there is no mechanism for doing this; cars with expired MOTs cannot be put back on the road; the system will not let cars with expired MOTs be taxed; not all insurance companies have said that they will cover cars with expired MOTs - all of which could mean some people unable to get to and from work, hospital, childcare, schools, etc. with catastrophic effect on their employment and finances.
Some cars with dangerous defects will have their MOTs extended and lives may be lost - when that dodgy balljoint fails or rusty brake line ruptures..
For many, cars are a necessity and ensuring that they can be on the road is a pretty critical part of society's running.
SteveW