The union is interested in keeping the union afloat. If the union sees no other opportunity it can become interested in keeping the company float, but you have not made the case for your statement to be a truism.

Consider an actors union — actor unions have famously walked away from companies without much regard for the longevity of the company they walked away from on numerous occasions. They know in that line of work there is always another company looking to hire them, so there isn't a whole lot of incentive to care about individual companies.