Customers are the ones that continue to pay. If they continue to pay they will likely receive maintenance from the devs. If they don't, they are no longer or never have been customers.

It would be interesting to see if there could be a sustainable OSS model where customers are required to pay for the product, and that was the only way to get support for it as well.

Even if the source was always provided (and even if it were GPL), any bug reports/support requests etc. would be limited to paying customers.

I realize there is already a similar model where the product/source itself is always free and then they have a company behind it that charges for support... but in those cases they are almost always providing support/accepting bug reports for free as well. And maybe having the customer pay to receive the product itself in the first place, might motivate the developers to help more than if they were just paying for a support plan or something.