Not only are security bugs an issue, as mentioned, but what about maintaining compatibility with changing platforms? Who will fix the bugs when future versions

Not only are security bugs an issue, as mentioned, but what about maintaining compatibility with changing platforms? Who will fix the bugs when future versions of Windows, MacOS, and Linux introduce growing numbers of 'minor' incompatibilities, and maybe major ones? What about changes in protocols, such as IPv6 (though mail protocols are mostly mature)? If Thunderbird isn't updated, it will slowly die of bit rot.