A side-effect of using SPF for the first part above is that sender
address rewrite generally suggested as a solution for forwarding
doesn't fit into TBR.
SRS is only one way how receivers can forward SPF FAIL protected
mails to third parties checking SPF "again" (i.e. at a point where
the "border" IPs permitted by the originator cannot help anymore).
Receivers can use other methods to get a similar effect, they could
whitelist "trusted forwarders" (e.g. in combination with an SPF HELO
PASS), they can "pull" their mails with POP3 from their old address,
they could also "pull" them with TBR if the originator, ex-forwarder,
and next hop (new address at third party) support TBR.
As far as SPF is concerned they could also use avian carriers... :-)