You Samuel, should know best, that application functionality cannot be
isolated from general OS bugfixing. As we know, it often looks like:
application z needs functionality y that can only be implemented if
component x is rewritten and requires w to be fixed in kernel. You cannot
just "make z work", unless you wish to hackplement it.
So application or rather target-oriented development may look great on
paper, but in reality it is soon bogged down into general OS development,
that we try to do nowadays.
Finally, you should know that devs are doing here the things they want/are
interested in. How do you propose forcing them to work on something
different?