Well, lets say hypothetically Ubuntu decide to change something downstream and that requires changes to a lot of other downstream applications. In the same update, Ubuntu can easily add patches and change all those as well... The update goes out and suddenly the source engine doesn't work because there is no source code to patch, until Valve release a new steam engine build. Bugs like this popped up a lot when pulseaudio was new for anyone not using package manager packages, or using obselete ones. In a complete absence of source code this can get a lot worse
I don't think this will last, Valve will probably give up on Linux support because the operating system is more trouble than it is worth. The market share is tiny, and with a moving target for an ABI it means this disproportionate amount of users also requires more effort and time invested. To get real application support OSS devs simply need to focus on having a more organized ABI that isn't changed in the downstream at all. The newest LSB will iron out a lot of these problems, but the distributors seem keen to actively ignore them.