If a software signing key is compromised it can be revoked and a few weeks later the risk is only to people who don't keep their OS up to date. Further, exploited compromises are detectable, especially if exploited at scale.
If the backdoor crypto key is compromised, sure they can revoke it (assuming they manage to design a competent system), but all the sensitive information up that point is now available to whoever possesses the backdoor key. Unlike the software signing case, exploitation of the compromise is likely undetectable unless the attacker reveals their knowledge somehow.
The same is true of SSL traffic to a bank though isn't it? If a crime group is intercepted encrypted traffic and saving it, then the keys are stolen, they can decrypt that data.
But opponents of the OSB claim it will make communication with your bank less secure - how?
If the backdoor crypto key is compromised, sure they can revoke it (assuming they manage to design a competent system), but all the sensitive information up that point is now available to whoever possesses the backdoor key. Unlike the software signing case, exploitation of the compromise is likely undetectable unless the attacker reveals their knowledge somehow.