seems like your concerns are weight and durability, so heres just another option to mtb commuting - which i detest.

i bought a really nice example of a early 90s cannondale on CL and had a indestructable set of wheels built. i wear a small bag and dont do rain commuting so mounts[rack/fender] were not required. 60km daily and i average 35kmh. i jump up and down curbs ruthlessly. my bike is a 60cm and weighs 23lbs with ATAC pedals, full under-saddle bag[tools, tubes, inflator], spare tyre strapped under the bag and full water bottle.

cyclocross bikes would be a good option as well.