I know what you mean but do not worry too much about the DMF. It is not something that will break down out of the blue. However, you are right. There is an extra part that at some point will need replacement.
With regards the cost, if you are long distance driver, you can easily add more than 100K miles before needing any attention to the combo clutch/DMF. With a bill around £1K it will turn out less than 1p/mile. If you are short distance driver, you add more stress to the clutch/DMF but as you will do less mileage, the parts will run for longer time. Even if it lasts less 50K miles or so, it can still go for 10 years to reach that mileage. That will make it cost something like £100 per year. If you consider what silly money we pay for insurance and never get anything back, such expensive repairs seem reasonable because you will only to do them once before the car rots away.
The other thing is that a lot of the drivers nowadays, replace their cars far too often to experience any such big repairs. If something like that comes up, they will just move to the next car a bit earlier.