How often do you switch off the camera before changing lenses? better than nothing, for sure, but dust (and shorter flange distances) are a problem of ML.
And at the same time ML is the solution. DSLR also sucks dust in, yet the cleaning is more complicated (deeper mirror box, mirror needs to be powered to stay upwards)
...and eventually fly towards the sensor. It only takes a little bit of electrostatic.
Shutter being closed as the lens is off is an awful idea. One misaligned lens or adapter or one slip of a finger and your shutter is gone whereas an open sensor would've been fine as it'd be burried significantly deeper and much better protected by the light baffle around it.
And it's much cheaper and easier to change a sensor glass than a shutter. Also keep in mind that a scratch or dust on the sensor is not really such a big issue while a damaged shutter is.
I think a shutter is basically useless to change the lens. If there's dust or mositure to protect the sensor (glass) from, it will remain on the shutter. As soon as I'm done with lens change, allt his particles will not disappear in thin air, but remain between lens and sensor.
Also, all DSLRs have a closed shutter at the moment of lens change. Does it prevent any dust particle to finally land on the sensor? Not at all.
I rather prefer easy access to clean the sensor. That is a system downside of mirrorless (and interchangeable backs). The shutter is kind of false security and obican is right. In case of a defect it's an expensive repair. It doesn't matter if it's fingerprint or particle, the glass is easier to clean than a thin sheet of metal. You guys considered it's a marge larger hole in the body and less deep as well?
The lens mount is rather large and if your lens or adapter has a small protrusion, it might hit the shutter as it's much closer to the mount than you think it is. Even some old m39 lenses might be serious offenders.
Also something tells me that if the shutter closes as soon as a lens is removed and only opens back up if the connection is reestablished, it might get tricky to use third party dumb adapters with no electrical contacts.