Absolutely they do not "backfill the role with someone who they estimate to be the most similar to the person who left!"
They in fact back fill the role with someone they think should be in charge of the future of the position (which is very often in a radically different direction), or more often than not eliminate the role altogether.
By your estimation, when Steve Balmer retired, the board would have hired another Steve. No, that's not the case. They hired Nadella who is different.
I am absolutely saying Microsoft's board replaced Ballmer with someone "like" him. To wit (as I said): shrewd. Ballmer shrewedly wrung all he could out of Microsoft's monopoly. People who think that Nadella is somehow some new, kinder, gentler Microsoft CEO are drinking the Microsoft-branded Kool-Aid. Nadella, per the popular notion, is only "embracing" open source out of a shrewd response to being forced to by the market. Moves like the one that inspired this entire article and discussion are proof of that.
Microsoft still sells (almost) no hardware, PC sales are dropping like a stone, corporate sales are lagging (my Fortune 150 switched to Windows 8 a YEAR after 10 was released), and Microsoft has to monetize seats of Windows somehow. What's left? Collecting and selling user data and advertising. This sort of thing has to have been approved by Nadella. Is this the move of a "kinder, gentler" CEO who just loves open source and freedom? No, it's a move by a shrewd person who is desperately trying to keep his company relevant in the face of phones, tablets, clouds, and social media.
They in fact back fill the role with someone they think should be in charge of the future of the position (which is very often in a radically different direction), or more often than not eliminate the role altogether.
By your estimation, when Steve Balmer retired, the board would have hired another Steve. No, that's not the case. They hired Nadella who is different.