I would be shocked to an extreme degree if Apple hasn’t been maintaining a version of macOS for their A-series chips for several years now. There’s precedent — they kept the lid sealed on their x86 OS X build for half a decade before the Intel transition was announced.
So while I agree that updates to the Mini are being held back by a pending ARM transition, I don’t think it’ll run iOS. It’ll run macOS for ARM.
Per-core performance has been stagnant in the past decade. Apple already appears to beat Intel's Y-series processors in most things. Scaling the un-core parts up to compete on desktop will take some doing, but scaling up into the relatively restricted 15-28W range is much easier.
Most importantly, Intel charges a premium for ultra-low power chips. If Apple can make a chip that's 80% of the performance for a couple hundred less per unit, it's a great deal. Moving everything to a unified ISA reduces design and developer costs and increases total control (aka vendor lock-in) which also increases profits.
So while I agree that updates to the Mini are being held back by a pending ARM transition, I don’t think it’ll run iOS. It’ll run macOS for ARM.