Well obviously it isn’t necessary to pay them ‘appropriately’ (as I assume you’re suggesting that many don’t. It may also be the case that the cost of losing a key employee times the expected number lost is less than the cost of paying them all enough that they’re paid ‘appropriately’).

But I think the more important question is: who are these key contributors? It’s perhaps easy to identify some of them, particularly those who are louder, and perhaps it is also possible to identify those who appear important but are, in some sense, bluffing. But there are also likely those who are key but whom the system for evaluation overlooks. It’s probably better for those people if they leave as they may find an employer who can better recognise their talents (and leverage them and pay them for it). I don’t think I’d do a great job at recognising such key people even among those I work closely with but maybe I’m just bad at that sort of thing.