You have several problems once your skeleton vanishes. Your solution must solve all of them, not just the most obvious ones:
- Bones protect organs. So now your brain is damaged if you bump your head on something. Or your lungs collapse if you run into someone in the hallway.
- Bones provide mobility. Without bones, your muscles have nothing to pull against. So there goes your ability to walk, talk, etc.
- Bones provide shape. If you collapse into a pile of soft tissues, all kinds of problems happen. You can't breath. Your nerves are all pinched. Your blood vessels are all pinched as well....
- You are no longer making blood. Bones produce white and red blood cells.
So you need a solution that solves all four of these issues, not just the obvious shape issue.
Maybe there are magical potions that slow or delay the deterioration. This is the cheapest thing available, so it's all your lower-income folks can get. The potion halts the effects of your disease for a while. So poor people take it as often as they can, but not as often as they should. Monthly, maybe. They all have early-onset osteoporosis and/or leukemia. But they get the treatment often enough that they don't just die outright.
Middle class citizens can afford the treatment perhaps weekly. So their bones weaken over time, but so slowly that they don't really notice.
Meanwhile, the wealthy take this potion daily and never suffer any effects. They can delay the disease indefinitely.
(If potions aren't ideal, then these are glowing green liquids that are administered via glass syringes, painfully...)
The most wealthy people have some kind of magical cure. Maybe its a ritual that requires a couple of days in a complex machine that takes up a whole room. The inner workings of this contraption are a closely guarded secret. The facility that maintains the machine(s) is in some remote place. Hard to get to, easy to secure, protected. Think of a mountain-top chalet with difficult approaches (because the wealthy want nice views).
This machine uses the same kind of magical sources as the injection, but at vastly higher concentrations. The process takes up to a week. It is painful, arduous, and unpleasant. It leaves physical scars that mark the person (like the Polio vaccine scars).
The rich see it as both a rite of passage and a barrier to entry. Only the wealthy can afford to book time in the Machine. Everyone talks about the experience in hushed whispers and vague euphemisms because they don't want to bring up the painful experience.
Those who can't afford it view the experience with an almost mystical awe. Maybe there are lotteries that the poor pay into, for the hopes of getting a week at the Machine.
The facility is run like an exotic resort -- clean, full of servants, well-stocked food and drink, all the amenities. And everything is designed to distract you from your daily round of treatments. Those who actually work on the Machine are treated more like priests during the Middle Ages than like modern doctors -- they know things and command respect by the virtue of their knowledge.