What Would Happen to an Index ETF if the Institution That Administers the Index Went Bankrupt?

As I understand it, some ETFs track an index that is maintained by a specific institution. For example, the SPDR S&P 500 UCITS ETF tracks the S&P 500 index, which is maintained by Standard & Poor’s.

So, if the company that manages the index were to fail (e.g., if Standard & Poor’s went bankrupt) or simply stopped maintaining the index, what would happen to the linked ETFs? Would ETF providers simply switch to an alternative index based on similar principles? Or would the ETF collapse? Have there been any historical cases of this happening?

Thanks!