GPL2 doesn't prevent a closed fork of Linux. It requires that if you distribute your closed fork outside of your organization, you must also distribute the modified source under the GPL2 license. Large tech companies run their own forks of Linux without providing the source.
Linux had many reasons for its success, but overall governance was definitely one of them. People felt free to contribute meaningful code in the knowledge that the GPL would “defend it” against a future as free fodder for corporations. Corporations treated it with kid gloves because they were scared of the GPL, and this left the ecosystem free to breathe; and Microsoft’s all-out war on the “communist” licence effectively solidified a front of motivated contributors in a way that the existing BSD efforts had never managed to.
Exactly. For corporations, it was like the guaranteed mutual destruction which keeps peace stable. They felt safe to contribute because their competition could not steal the contribution from the communal pot.
A similar setup is needed for the SaaS age. AGPL is a good step but is problematic for corporations because of the patent grant clauses, the way GPL2 is not.