> Never understood that argument, how is it different from your base salary, isn't it a similar mirage? Per year you get XX USD in RSUs vesting, you don't have to consider granted RSUs as being "yours".
If you can/do get an offer that replaces your outstanding RSUs and you're willing to move on, then none of this really matters - you are going to move on and the whole conversation is moot.
However, RSUs are typically based on looking back at previous work, not forward like a salary. If you did a ton of work at the beginning of your career and earned a huge stack of RSUs, you've sunk a cost... but one that is easily recoverable simply by waiting. Even if your output drops.
The biggest problem is that people are very often more productive and energetic in this field at the beginning of their careers, so much so that big companies don't even want to touch older people - built-in Silicon Valley ageism. And that's its own whole can of worms...
So after a few years when their productivity begins to slide and their life priorities change, they get married and have kids and buy cars and homes and pay for private schools... and getting a new job becomes more difficult by virtue of ageism and the sheer volume of outstanding RSUs a company would have to offer you making you a very unattractive candidate, people find they literally cannot tolerate the risk of moving on.
I've heard stories of people willing to accept fates such as being "roofed" (see Hooli on Silicon Valley) for months at a time looking for a new position internally while learning to be a world class barista in the employee lounge, simply because by sitting on their ass and doing nothing, they can afford to buy a house next year, but they can't find a new job because nobody is willing to give them the half a million they feel is owned to them as what is essentially back pay. It becomes a working pension, albeit one where the end goal isn't funding retirement, it's the freedom to move on. This is so common a phenomenon it has a name: Golden Handcuffs.
So yeah, some percentage sunk cost, some percentage risk tolerance, some percentage boredom tolerance.