A court won't force people to comply with the terms of a license. If they don't comply, they just don't have a license and can be liable for copyright infringement. A court can then enjoin them from continuing to distribute the product, which may be incentive for them to reach an agreement on a commercial license or it might induce them to modify the product to not use the infringed code. And a court can force them to pay damages, which will be the cost of what a commercial license would have been. However a court won't force them to release their code under the terms of the GPL. Even if the GPL were a contract, which it's not, courts just don't order specific performance for breach of contract.

As you mention, though, some potential defendants will determine it's just cheaper to just disclose the modified code and walk away.

I agree, a real, thorough test of this in court would be a great precedent.

And yes, if you refuse source to your customers that's where you are - software copyright violation. Just the same as if you did this with proprietary stuff. Someone with standing would have to start the ball rolling.