If I'm doing my job properly as a manager, no one should ever be indispensable.
Highly valued? Sure. I want to build a team where everyone is exceptionally valued.
But if anyone ever becomes indispensable, I've failed in my job as a manager.
Why? The hit by a bus factor. That wonderful employee who loves me, who I love... can still get hit by a bus. Can still get sick. Can still have a loved one die. Can still have a relative offer to pay all expenses for a once in a lifetime six week world trip.
If I have any employee that I can't keep my team running without, even at zero notice, I'm not running my team well.
It may suck. It may be sad. It may require some juggling I'd much rather not do. But any indispensability means I've done my job badly.
This means, if someone quits with zero notice, I can handle it.
At that point, it's actually a good thing anyway. If they're so pissed off that they'd statement quit, I don't need them in the office, poisoning others, dragging their heels through their short timer's disease. Let's get them somewhere where they're happy and get my team of great people back doing great things. We'll live.
Strange thing? When you have a well run team that you can already be confident in, people rarely statement quit anyway. For some reason, they don't seem to feel the need. Imagine that. And when they do? You've got it handled anyway.