Steriods are not good long term for oesteoporosis unless supplements are used too (for humans) but in a dog even long term is not a long time so and if the vet thinks that may be the best course of action I think you'd have to take it. You want to ensure your dog as quality of life over quantity and steroids are definitely a wonder drug.
I don't know of this condition but funnily enough I did pick a leaflet about canine osteoarthritis in the vets this week.The leaflet says treatment is focussed on reducing pain and inflammation, slowing the progression of the disease, facilitating the repair of damaged tissues and maintaining or improving joint function. Treatment involves weight control, dietary changes, controlled exercise and physical therapy, anti inflammatory and analgesic drugs and disease modifying agents.
I know with my MIL's osteoporosis she is on anti inflammatory and analgesic medication, she also takes a low dose of prednisolone, and she has a calcium supplement and a Vit D supplement to try to slow down the process of bone density destruction.
I know my answer is not specifically about a dog but I'm sure the treatment of bone destruction would be similar.