If there is a documented problem due to their negligence it is their responsibility to fix it. It doesn't matter what kind of warranty they offer or if it wasn't reasonable etc...or if you purchased or filed for a warranty -- or not. This is a defect right from the factory, which is a different realm than standard failure of equipment covered by warranties.
Individual action wouldn't be of much use and too expensive, impractical. However, a class action suit would be possible and would probably be effective, given the widespread nature of these issues. The customer has a reasonable cause to expect a non-defective product. If we're talking about a defective electric toothbrush that a manufacturer refuses to replace, that's one thing. However, at $7500 and beyond, it would be considered relevant in a court of law that there were widespread, systematic failures in the products produced by Jersey Jack Pinball or Stern Pinball.
Through the lens of a class action suit, a reasonable settlement would be for Stern or JJP to provide populated playfields to those who could (conclusively) prove that their PF is suffering from the issues discussed. Installation would be a nice touch from the manufacturer, but the judge would probably be content with the company offering a fully populated PF ready for install for each effected party.
Merely providing a PF (that they are making profit from!) does not even begin to solve the problem that was inflicted upon unsuspecting customers with reasonable cause to assume they were receiving a non-defective product.
These companies have liability...but it all depends on how far we - as a group - are willing to take this. Often, the legal costs and headaches just aren't worth it. This is what many smaller companies such as Stern or JJP who exist in a niche market...are depending upon.
But their legal asses are potentially swinging in the wind here.