Managers are just employees as well who also pick projects that will benefit their careers. Why work on a small thing with a few million users where you will never get promoted since you are already at the top, when you could just move to the next growth ship where lots of people are hired so moving up the ranks as a manager happens naturally? To me it seems like this culture seems to go all the way to the top. It is not like Sundar got promoted so quickly due to his work on an obscure product.

Yup. Can't get that promotion if you are on the wrong product/team, that's what I heard at least.

Why support a dying product? Pivoting is a key part of startup success. Why can't large companies recognize a shift in user needs (after a decade) and decide that something is no longer valuable?

Ultimately, you really are never going to see innovation if the cost of launching a product is "staff the product literally forever, no matter if it is losing money or no longer aligned with priorities".

If the project is a dead end, and nobody knows how to change that, it sounds like a good idea to let it die.