If Apple can't make small changes easily, they won't be able to respond to security vulnerabilities either. This is, after all, what security updates are all about.
But "equivalent economic activity" seems a bit weak. A butler or security guard can be paid to open the door to visitors. So that's "economic activity", right? It doesn't mean they're not required to respond to a search warrant. And legal discovery can be very expensive.
Writing a simple software patch and recompiling doesn't seem all that hard. Rewriting the entire OS is a major undertaking. It may require passing new laws, but it seems unlikely that we'll end up with a bright-line rule where touching any line of code is forbidden. More likely it'll be a gray area where the line is drawn somewhere in the middle.