And as proven by FOSS drivers that eventually got purged from kernel tree, having source available is meaningless if everyone just lets the code rotten to the point of being purged.
As for pressuring vendors, the source code is available, and the license allows them to never upstream, it isn't as if Joe and Jane know what that is when buying a new phone.
Two. Linux? Who's counting anyway.
Eventually the community will write their own drivers (as has happened with Mali) and the respective vendor will have a reckoning internally (like what happened with AMD's GPU driver) whereby those advocating for wasting money on out of tree code leave the organization with egg on their face for bilking the company out of significant R&D cash.
It is very hard to claim that its worth it to develop a proprietary driver when a comparable performance driver already exists for your hardware in-tree. The in-tree driver will almost certainly be easier to use & break less often than out of tree code that the kernel devs don't think or care about breaking.
Its a step forward in kernel upgradability, but it also takes the pressure off vendors like Qualcomm and ARM to upstream their patches. I hope the kernel continues making significant changes that break out of tree, proprietary drivers.
The harder it is for these players to act in bad faith, the sooner they'll collaborate with the kernel developers and mainline their code.