I see your point as well, but I think that with the price of SUVs in general falling through the roof, it may be better to take a more personal perspective.
Meaning, your truck is worth more to you than its market value. If you had to replace it at market cost, it would be very difficult. You wouldn't feel the same way, buying a cheap ML from someone else, knowing what you know now. Knowing how you have cared for your ML is worth a certain amount of confidence that you just can't have in someone else's car.
The bottom line is that if you sell it, it is only worth the market value, but if you keep it and bring it up to par, it's worth a lot more (to you). It's a fantastic vehicle, with the exception of all of the sensors that tend to fail randomly. You'd be hard pressed to find a vehicle with the same mechanical quality/comforts.