As we also learned in Asiana 214 News briefings, landing gear assemblies are frangible. They are designed to break in specific places when overstressed to avoid more excessive damage to surrounding structures. In the case of the 777 the main landing gear shearing off probably saved fuel tank integrity and prevented a fuel-fed fire.
In this case I'm not sure how the wheel itself could roll along as it is seen in the one video, when the gear strut was pushed up into the avionics bay. To me the bouncing wheel seemed bigger than a nosewheel (but I agree it is hard to judge without any reference), but in the photos of the aircraft at rest one can see all 4 main gear wheels attached.
Question for 737-7xx pilots: what is the clearance of the engine nacelles with the nose gear completely collapsed? I looked at some drawings of the (slightly longer) -800 and it seems to be very little (if any). So given the shorter length and some compression of the main landing gear on "touchdown" I assume the answer is "less than zero", i. e. the nacelles are expected to scrape in this case.
Would one expect engine damage from scraping the front portion of the nacelle like that or would the air inlet just deform and protect the engine proper?
Bernd