Critical thinking, IMO, would lead one to believe a subscription is inevitable. Apple most likely spent a ton of money on AI and expects a return, as do the shareholders. Seems like virtually everything today is being sold as a subscription. Looking at other AI solutions like ChatGPT and you can see where they lure you in with their free tier and then upsell the premium subscription. In particular this is the business model ON Apple devices, you get the free tier then can upgrade with ChatGPT and Apple gets a cut of that I assume. I'd almost say that's Apple already charging an indirect subscription seeing how tightly ChatGPT is in Apple Intelligence. Plus it's not like Apple doesn't have their own myriad of subscription services obviously, and these are in some quarters their fastest growing revenue sources.
Certainly all of this is based on ONLY rumors and speculation, but honestly I would be hard pressed to think Apple would NOT charge for some sort of premium tier of its AI in the future. My feeling is that it better be useful and not just fluff for consumers to pay.