Are you saying that if you can pay someone to develop their proprietary code for a price you can necessarily find someone to accept the same amount of money for the same work but open sourced? That doesn't seem true. Those are not equivalent offers.
If there isn't then the original commercial company was obviously never viable either.
Since an entity getting payment for maintenance can neither expect to collect a monopoly rent for the code, nor is required to pay one to anyone else, the cost of maintenance becomes the market price.
Especially when you start talking about a single person company, it could be as simple as the founder being sentimentally attached to the company and continuing it despite making less than they could with other opportunities, but that doesn't mean that if the founder gets hit by a bus someone else will also be willing to take a sub-optimal gig. Or a company might currently be barely worth running with $20k a month in revenue, but if there is a period of turmoil and lost customers in the process of open-sourcing/founder getting hit by a bus/whatever that now the customer base would only bring in $10k a month, it's no longer profitable, and the market fails to find someone to run the company.
I would assume some people (clearly a minority) would be willing to take a small pay cut when working on FLOSS.
E.G. could someone be paid slightly less at GitLab versus the same role at GitHub, if they believe in GitLab's principles