Push back by transit ASes is harder. This is what I mean by that:
A --- B
/ \
X Y
\ /
C --- D
C's link to D may be low capacity or expensive, so D would prefer it
if X would send traffic to Y over another route if possible. C can
make this happen in BGP by prepending its AS one or more times so X
will see the following AS paths:
A B Y
C C C D Y
All else being equal, X will choose the path over A to reach Y.
The simple answer here is that if the multihomed site receives a BGP
feed just like today (except that it's a read only feed) and thus
makes outgoing path selection decisions just like today, transit ASes
have exactly the same tools as they have today. But presumably, if
shim6 takes off many smaller sites that aren't comfortable with BGP
will multihome also, so this push back won't work as well anymore.
Creating a new way to accomplish this result is probably possible,
but not entirely non-trivial, and probably something we wouldn't want
to deliver on day one.