They are not obligated, but it is in their best interests to build a reputation of not breaking things.
I do think that Linux lacks a reputation of not breaking existing programs as much as it lacked a reputation of ease of use before Ubuntu.
Now, ease of use, and not breaking things? I bet that kind of reputation is worth millions of dollars.
Even Linus agrees about it, when it comes to the kernel breaking userland programs. The kernel developers have no control over userland programs. Nonetheless, the kernel developers try very hard not to break userland programs.