The cost is very high but it's only getting higher with time. People have known that SHA-1 was weak and deprecated for much of git's existence. Doing the switch in 2010 would've been painful, doing it now would be orders of magnitude more so and I doubt it'll get any easier in 2030 unless some other SCM manages to overtake git in popularity which seems unlikely at this point.
Unless Linus really believes that git will be fine using SHA-1 for decades to come I don't think it's very responsible to keep kicking the ball down the road waiting for the inevitable day when a viable proof of concept attack on git will be published and people will have to emergency-patch everything.
Unless Linus really believes that git will be fine using SHA-1 for decades to come I don't think it's very responsible to keep kicking the ball down the road waiting for the inevitable day when a viable proof of concept attack on git will be published and people will have to emergency-patch everything.