This process reoccurs for a reason. Understanding that helps mitigate the loss, and possibly also helps new companies avoid the problem.
Basically it happens when people start needing money for life, and that need becomes more pressing than idealism.
Consider the start of your career, maybe you came from school or college, probably you had _very_ low money needs. You may have had subsidised living (living with parents, student loans etc.). You live in a world where money isn't as important, you have no obligations, you are as idealistic as you'll ever be.
At this point your world is "open source" - everything should be free, make the world a better place. It's not hard to find like-minded people, join companies with like-minded founders.
But life happens. You have expenses. Relationships, kids, obligations. If you have employees you have to make payroll. Some might call it "growth" - some might call it "growing up" - really its just discovering (and losing) some of those subsidies you took for granted.
Crucially it happens at different rates for different people. So inevitably there's friction - this isn't the company I joined - and so on.
It's hard for employees to understand the pressures that come with being an employer. Pressures that lead to decisions an employer would rather not have to make. Pressure to make payroll. Pressure to somehow make it work. Seeing the car park and realise the number of people dependant on making that work.
To an employee every decision looks like maximising profit. And in some companies that is true. In others its about maximising income, income to pay everyone, income to keep the lights on, income to build reserves to weather the storms.
Employees have the luxury of quietly looking elsewhere. Once they're set then can simply leave. Employers don't have that luxury.
Unions, especially unions belonging to one specific business, not industry wide, are not a bad thing. But a seat at the table means understanding the responsibility of that table - and the need to satisfy the needs of all, not just your own. I've seen unions be a huge asset, I've seen them destroy factories and industries.
Yes companies pivot from open source all the time, because yes they need to "capture" value - because at some point costs, and life, catch up and unfortunately "giving it away for free" doesn't really pay the bills.