Honestly, I think the claim that the free market would solve this is so outlandish that the burden of proof is on those who believe it.
Maybe, just maybe, in a dense population centre like London their needs would be met by a few specialist 'accessable' stores. But what about some rural town of a few thousand people?
It isn't that outlandish that the market will sort it all out. I doubt anyone is going to be unhappy if business get a bit of a prod to remind them that wheelchairs exist, but the idea a free market would ignore 2% of their potential customers is just not true. Greedy capitalists have incentives to be thorough; 2% of the market changing hands is enough to get the attention of any CEO.
Most businesses would notice 2% of their customers disappearing, let alone 2% of the broader market.