Actually, I think the "in business, NOBODY is indispensable" saying isn't 100% accurate. ANYONE can be REPLACED, but in my experience, when top tier talent leave, their replacement is rarely at the same level. Over time, as enough of these people are "replaced", you just see the long, slow decline in the ability to get complex stuff done in a reasonable amount of time with solutions that are as creative/appropriate as they could have been. Everyone just adapts to the new lower standard of output.
I actually think this pattern is one that causes the slow death of large companies over time (and the replacement with smaller, more nimble and, frankly, talented ones). Ironically, they all SAY "people are our most valuable asset" without actually realizing IT'S TRUE! A friend of mine once told me he felt companies viewed employees as "liabilities"...and I have come to believe he is correct <insert Optimus reference here>
Oh and those smaller, more nimble and talented companies I mentioned....guess what happens when they get big...circle of life.