Yeah, performance-wise the A12 is probably as fine as the A9(X) on iOS 12. Like I said, I have praised Apple for not destroying my 9.7-inch iPad Pro, at least performance-wise. Like I stated, battery life-wise it is significantly worse, but it isn’t unusable. I have stated that even the keyboard lag that my 9.7-inch iPad Pro gets is completely negligible on iOS 12. Performance can reliably be considered near-perfect. I have repeatedly praised Apple for this.
I reckon that is what happens with your Mini 5, and what would happen with the Xʀ, at least for now. Like with my iPad, performance would be fine, and battery life would be significantly worse but not unusable.
My 6s on iOS 13 is significantly worse than negligible, however, to the point of being annoying. Like I said, I have higher expectations, because every device I use is one that works perfectly. The only reason why I still like my 9.7-inch iPad Pro and I still call it my favourite iPad ever, is because even if it was forced out of iOS 9, its performance is perfect, and battery life is half-decent enough. Yes, it probably saw a 25% decrease, but I don’t need it and it isn’t too bad. Would it be better on iOS 9? Yes, but it wouldn’t be twice as good, and that’s important to me: it is close enough considering what it could have been. What if the iOS 9 bug had occurred now? I’d have to say goodbye to the iPad, battery life on iPadOS 16 is probably abhorrent. Many reports of the 1st-gen iPad Pros have mentioned that it struggles severely with battery life, getting 3-4 hours. Compared to that, my 9.7-inch iPad Pro is amazing. I have always looked at the glass half-full in that regard, ever since I saw the difference.
As far as performance drops on newer iPhones goes, I am skeptical. The A9 was widely considered a powerhouse when it was launched, and yes, it isn’t as poor as an A6 on iOS 10, but it is significantly worse than the verbiage back then implied it would ever be. Battery life rendered it unusable, and its efficiency back then was cited as one of the reasons why, perhaps, we wouldn’t see this battery life degradation. I expected the Xʀ with its battery size to be the next step, but it wasn’t. Performance is as good as the A9 or better (three to four iOS versions in), but battery life has suffered.
Is the M1 enough? A desktop-level chip? Is the A16 enough? We’ll see, but I’m not overly confident. I hope so. We are guessing at this point but I fully agree with your assessment: performance-wise it will probably be fine, and battery life-wise... it might be significantly worse, but considering the charging speed you mention, the ability to fast-charge, and assuming that maybe it is better than the current standard, long-term users of these devices will probably be in a much better position than A9 users are today. Some users like me will remember and say “do you remember the battery life an iPhone 13 had back on iOS 15?”, others, perhaps as rightfully as me, will say “I do, but do you remember how A9 devices suffered back on iOS 15 and iPadOS 16? Well, this iPhone 13 is far more usable than that”. I hope that’s the case. The device is too new to know at this point.
And this goes back to the OP: should this be the case... maybe everyone can care a little less about maintaining battery health even while updating their devices. I look forward to the day when a fully updated iPhone is as good as my 6s is, 6.5 years later on its original battery. With the obvious negligible decline due to very poor battery health, but fully working. Performance-wise we are getting closer, today’s performance on 4, 5-year-old devices is a far cry from the A5 and A6 era and that is a good thing.