IF the Colts sign Rivers...JB will be gone in a trade. That is what is best for both parties...and they will not carry two QBs with $35-40M in cap space tied up. JB likely doesn't have a ton of trade value...but the trading team is only taking on $13M (which isn't a ton in the current QB market)...so he's definitely moveable...even if it requires some creativity.
JB is currently a sunk cost...and whatever dead cap space is ultimately attached to him is just that. But by trading him...you would be freeing up $15-16M in cap space...which can be used to get a better stop gap. So whatever additional cap space (above the $15-16M you saved) is what that new QB costs.
I would rather have a band-aid than the current open wound. Besides, Rivers is not meant to be the ultimate solution...he is merely a stopgap and mentor for whatever rookie they draft.
Reich and Sirianni came to Indy to coach a top tier QB and win. This roster is likely good enough to compete...but another season of JB offers neither top tier QB play or the real chance to win. Rivers might...and that's really all they can do at this point...until the next guy is ready to take over.