[Mondaveo]

It's not a sealed unit... requires complete removal from the car and specialist tools to dismantle and reassemble, yes, but it's not impenetrable. They can be serviced and repaired without replacing the whole unit.
Source: I had an internal sensor replaced to fix a gearshift malfunction. The specialist took apart my existing gearbox and changed out the sensor, they certainly didn't bin the whole 'box and charge me for a new one (although with labour accounting for the majority of the bill, it might not take too much for slotting in a new gearbox to be the more economical option!)...