It (transition from old and busted glibc to a imaginary shiny and sexy replacement-for-glibc)
could probably be handled in a similar fasion on how the X.org developers are handling the
transition from XCB to XLib.
First they tried to go with XCL, which was a XLib compatability layer for XCB. It worked for
the most part, but a lot of the more oddball features of Xlib proved too difficult to
replication on top of XCB.
Now they are doing the Xlib/XCL approach, which is to have Xlib-guts being slowly ripped out
as applications gradually migrate to XCB, while having the Xlib actually running on top of
XCB. (or something like that) Using this approach they can acheive 100% binary compatability
and for programmers of applications/libraries using xlib they can start using XCB stuff right
away without a total rewrite.
Also there is probably some answers to be found in the Linux-binary emulation features present
in other operating systems like FreeBSD, AIX, and Solaris.