The main issue is there isn't enough time to implement such a solution. You have to develop this new system of handling API keys, communicate this change to customers, develop a process to migrate users, and figure out what to do with people who paid for an entire year of access 3 months ago. All of that is a herculean task for a small dev team to accomplish in 30 days.
That's not even dealing with the fact that this process would be difficult for users to actually use, and may run afoul of Apple's app store rules.
While that solution may be appealing to tech-savy end users, it's completely untenable for a popular app, especially given the tight time window required.
