This is a really great point and something that I keep in mind when considering my current job and future offers. Its really easy in a big org, to stick around for a long time, feel valued and wanted (intra-company professional network, regular raises and promotions etc.) and think that that is it, that its a good enough life. I see a LOT of people like that and I've always wondered if they could possibly move to another company and be as happy as they are in their current position.
I'm still figuring out ways to not get into a rut like that...
If someone can carve out a spot and be really happy somewhere for a decade, I say more power to them, but man is it a risk.
The longer you're at a big company, the more you get used to it, and the harder it is when it's time to move on. I worked at a big SV company for ~4 years, did extremely well, stack ranked in the top 5%, got raises, my last review I got a "critical talent stock award" -- it's easy when that's happening to imagine yourself there forever.
Then over about 6 months my division was divested, my manager was replaced, and a bunch of us in the old regime got laid off at once. It was heartbreaking, I was in shock. I bought my first house there, had 2 kids as an employee. Transitioned really to a fully-formed adult there. The work was interesting and I had a wonderful team. The job was a part of me, and I went from blessed top performer to the soup line in a matter of months.
But that's what happens when you don't manage your career really hard. If I had been smart/ruthless, I would have moved diagonally to a new company and started making a name before they had the chance to make a move on me. But I fell for the siren song (and vesting schedule) of the Big Corp. This isn't an unusual story.
Its true, although it doesn't hurt to be a step ahead of the game. And to be sure, one can usually see the warning signs if one is careful enough: lack of growth in the division, change of management, changes in the market etc. When you're in a good spot, you often tend to not look as hard. If you can pre-emptively find a better position instead of waiting until you are laid off, it gives you a lot better sense of security when negotiating/selecting the new role etc. And for many tech workers like myself who are immigrants, being laid off creates attendant immigration issues which are a huge headache to deal with.
I'm still figuring out ways to not get into a rut like that...