after all these years the insurer basically gave them the ownership back, I got it
Hmmm... an interesting question.
As a former (tax) accountant, I wonder about the accounting involved in the original loss. The insurance company paid the museum for the loss and, in return, created an asset on their books, the asset being the value of the rights to the stolen artwork. Over time, I suppose the insurance company would depreciate that value since the odds of recovering the stolen artwork would decrease over time. Still, there should be some residual value.
After 60 years, the residual value of the stolen artwork would be negligible, but they could not write it down to zero. So... I suppose donating the negligible rights back to the museum could be a publicity thing. Or, maybe more likely, an incentive for the museum company to keep insuring with them. And that would allow the insurance company to clean up their books.
It'd be interesting to find out how often this occurs in the insurance industry. I'm not even sure what to google to discover that!