Why is cartilage replaced by bones in a vertebrate adult?
Bones are complex organs, made of more than just a structural element. Cartilage is just a tissue, usually devoid of nerves and blood vessels. 

Chondrichythes live in water (ἰχθύς ichthys is Greek for 'fish'); the difference in specific gravity and buoyancy is considerable, and cartilage is not robust enough to mechanically support your body out of the water. 

Vertebrates have evolved to have a bone marrow, which is part of the bone. If the structural element of the bone (the dense mineral matrix) was replaced by cartilage only, then the hollow structure would make things even worse.

Additionally, cartilage has about the same density as water (Pan et al, 2016) whereas dense bone is 60-90% more dense, which means extra weight pulling aquatic animals down.