Fine. Please ignore the rest of this comment because anything you don't agree with must be a troll. Continuing with the hypothetical example of TTY changes to a BSD-based operating system...
In order to coordinate changes to the kernel TTY code that potentially impact thousands of userland programs, you would need a team of developers and testers to go through the code looking for problems. First, you fix problems in the the core userland programs (aka. the "base system"). The source code for everything is under /usr/src, so you can start with:
Once you have a list of potential problematic ports, you send out a notice to the ports maintainers and users of the -CURRENT branch asking them to test the affected ports against the experimental TTY patch for the base system. Some ports, such as Emacs, may rely on the old behavior, so they will need to be patched. These ports will require patches in order to make them work with the new kernel. Other ports may not need any changes at all.
Once all the changes are tested and reviewed (kernel, base system, and ports), the combined patchset is applied to the -STABLE branch for inclusion in the next stable release. All of this development and testing is costly, so hopefully the kernel changes were worth it :)