Have you noticed the power of open source software? Have you made efforts on customizing an open source project licensed with a GPLvX license? Have you based your business on an open source project?
The question now is: why you do really think that maintaining a private dead-fated branch protect your business better than mainlining it? Even standing at the fact that the private branch code must be released public to not violate the GPLvX license?

Open source is going to be more pervasive than every wise man could have thought some years ago pressed by the free of charge licensing first but then from its own robustness.
Following the Linux kernel example, important vendors of hardware components are changing their point of view on making public available the documentation needed to develop on their hardware platforms, because they understood that this means increased chances to hit the market with products in a shorter time and consequently bigger sell volumes and longer chip life cycle. In the past they decided to follow the “Do it on my own” way having private branches for their own BSP but this approach is faulty in the way that need a lot of efforts maintaining the resulting heap of code over new versions of the kernel: resolving conflicts when new code cross the private one, backporting drivers to support new devices etc.

These are hard tasks to redo from scratch every time a backport/upgrade is needed, rising sometimes even the feasibility problem. A community driven approach instead could eliminate these extra efforts: a mainline BSP resides in every single version of the kernel since its merging, getting the advantage of every new functionalities developed in the former versions of the kernel and the feedback from subsystem maintainers if external modifications touch the BSP merged code. The reached spread of merged code takes advantages also from a bigger testers/maintainers army which can evidence hidden bugs  before these can affect the product users. All this, applying naturally the kernel license.

This example stands also for all other open source projects.