The biggest problem to any sort of complex artificial limb is the fact that they would not be able to self repair.
We cause small amounts of damage to ourselves aaaall the time but barely notice as it heals in a few days, these limbs would need regular MOT and costs associated.
Can't afford your car MOT, well can take a bus for a couple weeks till pay-day.
Cant afford your LEG MOT..........well now, can't put your old legs back on
While that's true, I think it wouldn't usually be as large a problem as you're arguing it would be for a number of reasons:
When they're first available, the cost would be high enough to make them available only to people who are rich and so the maintainence costs wouldn't be an issue for almost everyone who had them. There would be some cases of people who were borderline rich enough to afford them and they might find maintainence costs a problem, but that wouldn't be the case for most users. In time, they
might become much cheaper and thus available to people who aren't rich. If that happens, maintainence and replacement costs would also decrease by approximately the same proportion so it still wouldn't usually be a problem.
These predicted artificial limbs should be much more robust than natural limbs. Natural limbs sustain small amounts of damage all the time, but most of the forces that cause damage to organic tissue wouldn't cause damage to advanced artificial materials. Complex metal alloys, carbon fibre, etc, are far more resistant to wear and tear than cartlidge, tendons, etc.
These predicted artificial limbs should probably have various models at various costs. You use cars as an analogy, so I will too. The insurance, maintainence and repair costs for a Ferrari LaFerrari are vastly higher than those for an entry model Ford Mondeo, but a Ford Mondeo still does the job well.
While you wouldn't be able to put your old legs back on, if the systems are designed well you would probably be able to put a cheaper pair of legs on until you could have your more expensive pair repaired. A loan pair from your insurance scheme, for example. Or an old used model you picked up cheap at some point. In that case, these wouldn't be like cars - you wouldn't need much space to store a backup pair and keeping an unused pair in working order should be very simple and cheap.
So I think that the problem you refer to would exist, but not on the scale you refer to. It would affect some users, but not most.