If you purchased music before the switch, that does still have DRM. (As an aside, you can get rid of it by setting up a VM with an old version of iTunes and Requiem.)
If there are literally no assets there's very little that can be done though, you could threaten the owners with jail but what can they even do if there is no money? If there were a law it would almost necessitate something more like unemployment insurance where companies are forced to hold insurance to deal with this in a predictable way if they shut down or close a customers account.
Actually, unemployment insurance might be a pretty good analogy. If a company goes out of business and you get laid off as an employee you get unemployment mostly paid for out of money the company paid to an insurance policy. In this case instead of being triggered by unemployment it's triggered by loss of access. In the same way, if a company is closing a lot of accounts their insurance rates go up.
They'd be pretty massive liabilities and would significantly eat into the recovery for other creditors, which could have effects on Valve's ability to issue debt, or might just mean that whenever Valve issues debt there's a clause making that debt senior to any future we-terminated-your-account liabilities.
How does that work? Surely that’d have to be something agreed to by both the lender and the customers?
I suspect any law would state it has an availability period before refunds are not eligible.
It is perfectly reasonable for a company to be liable for refunds if they want to deliberately take products or features away from users. In order for a company to avoid this liability, they must provide a way for those features to still be available to users, even if the company no longer supports the product.
This is pretty easy and straightforward to legislate. A media company that provides distribution of individual works (like Steam or Apple, as opposed to subscription services like Netflix), must provide a method for users to easily and automatically back up all purchased content. A company that runs servers necessary to the use of a program (e.g. matchmaking servers for video games) must provide the server executable, in a form that can run on currently-available commodity hardware, and must allow the client to select a privately-owned software. Companies that don't meet these requirements would still hold full liability for refunds if they remove product features later on.
This does not seem like an enormous leap to make.