Too simplistic. Some people are trapped - the availability of even just one alternative job with chance of improvement is not a given. It's question of age(ism), ableness to move to another city / country, and luck.

Or put differently, we're not all around 30 anymore, live in the bay area, and might have family matters, e.hg. a wife that doesn't want to quit a job she loves so you can maybe improve your happiness by 10%.

Also, companies change over time. A great place can become hell with just one bad superior getting hired and poisoning everything.