Your perception of the kernel development model is broken: It's not that 2.6.30-rc5 (or 2.6.31) hits the desktop of Joe Plumber the day
after publication on http://www.kernel.org/ of distributions. That's the job of distributors (and it's the same in the
BSD world - they are distributing the complete OS so they must be compared the Linux distributors).
Assuming that "kdesu" is broken, there is plenty of time for "kdesu" to get fixed (and pushed expedited downstream because it's a bug fix!).
Maintaining bug compatibility is plain simply not worth the effort (and it is IMHO violating any sane open-source development model
where it's at least possible to *fix* bugs and not maintain an enormous amount of bug compatibility code - and maintenance effort - for ages.
Why else should Windows needs exponentially more resources with each release?).
If someone wants eternal backward bug compatibility, please *do* it - but do not complain to others or even ask them.
Especially not for some random buggy app which just happen to not trigger a race condition.