Let's say the FBI wins the first case.

Then they request a warrant that requires future iOS versions to permit similar access.

By what argument would Apple fight that warrant? The previous case would have already established that such access is necessary, that Apple can provide it, and that the court has the legal power to force them to.

In terms of the burden on Apple, the staff time burden to write software that will run on 50 million iPhones is no greater than software that will run on one iPhone. Code is code.

In terms of damage to the Apple brand, the FBI has already argued that that should not matter:

http://www.nytimes.com/2016/02/20/business/justice-departmen...