The CVS repository contains an entire
    progression of EGCS source trees, from the time the project was
    started, until now.  Anyone wanting to start with the most recent
    tagged source tree, or any intermediate source tree, can just do
    merging work directly from that...

You're right that CVS helps here.  Making changes in EGCS today won't
interfere with the specific job of merging a certain change that was
present in EGCS yesterday.  But the issue at hand goes beyond that.

Right now, certain features in EGCS are mature and it's time to put
them into GCC releases.  But there will be more in the future.  In
addition to moving certain changes to the GCC release now, we need to
have a smooth flow of changes from EGCS to GCC releases over the long
haul.