Mainlining a driver means it will be immediately available to users of a huge number of architectures. It will be improved by other Linux kernel contributors. It will never have binary compatibility problems with the kernel, letting users update their software in peace. Maintenance costs are significantly reduced since it is no longer necessary to keep up with a free software project that receives over 20 patches an hour.
Maintaining drivers out of tree requires a few hours of testing and maybe some minor changes every two months (and that's only if you for some reason want to keep up with the main release cycle, and not just with LTS releases).
It certainly does not mean keeping up with 20 patches each hour.
People spend more time walking their dog during a week, than they would maintaining the out of tree driver every few months, if they changed to that as a hobby.
