But the truth remains, Apple ported macOS to the iPad, and then replaced the Mac with iPads in Mac form-factors.
...which worked really, really well for almost their entire product range, except for that one, shrinking, 1% niche that needed a big box'o'slots. Turns out, the iPad and iPhone were already doing tasks like image processing and video handling that would have been Mac Pro territory a few years back.
It turns out that it was the ideal form factor, after all, and that it was ahead of its time.
Depends what you mean by "form factor" - the "small non-expandable system relyng on Thunderbolt for i/o" does seem to have come of age with the Mac Mini/Studio. Maybe it would have worked better if they'd run it alongside an updated tower Mac Pro for a while - instead of letting the previous Mac Pro wither for a couple of years (it had been discontinued in Europe for want of a plastic fan guard).
However, the actual design of the trashcan "form factor" included the "CPU and two GPUs around a triangular chimney" concept (which led to the cylindrical shape) turned out to be a complete stinker. The market stuck with big, single-chip GPU cards that were 90% heatsink. Also, the elephant in the room at the time was that a lot of potential customers wanted NVIDIA and CUDA rather than AMD and OpenCL - which was ruled out by Politics rather than any technical issue.
OK, so the Mac Studio has somewhat redeemed the "small, sealed system" concept, but it rather depends on the low-power, system-on-a-chip (including a half-decent, power efficient GPU) design of Apple Silicon - which doesn't suit the traingular-core Trashcan concept.
Like are you really telling me that they couldn't have stuffed newer and more efficient i9 chips in that thing, just to not embarrass themselves?
No, because the problem wouldn't have just been the i9, but the single huge GPU that the i9 would have needed to make the system a contender. The trashcan design bet the farm on the CPU + 2 small/medium GPU formula.
Obviously, given that they managed to fit the i9 plus an OK GPU into the iMac - and a Xeon W + half-decent GPU into the iMac Pro they
could have made a small, headless desktop i9 or Xeon Mac, but the trashcan design wouldn't have cut it.
Real problam is, though, the "Mac Pro" market seems like a very low priority for Apple (probably because it is small and shrinking). They keep releasing a model, neglecting it for years, then completely change course and target a new market. Vis:
1. 2006 Original "cheesegrater" Mac Pro - entry level was relatively affordable (for what you got) Xeon/PCIe towern "pickup truck" for anybody who needed a powerful, versatile Mac. Top-end options were pretty high-powered (and pricey). Became abandonware after ~2010 (plus, there's a whole story about the original 32 bit firmware models getting obsoleted pretty quickly).
2. Trashcan - a non-expandable "appliance" for FCP and other OpenCL-optimised workflows. Interesting idea but - certainly at the time - not really a replacement for the cheesegrater. Never got a significant update (beyond dropping the original entry model and shifting the others down a price notch).
3. 2017 iMac Pro. Now, apparently all "Pros" obviously want an all-in-one and never have more specialised display requirements. Right. I strongly suspect this was originally meant to be the replacement for the trashcan - the famous Apple U-turn press conference came a just the time when they might have been sharing iMac Pro designs with key developers/customers. Pretty clear that - at that stage - the iMP would have been in advanced development while the "mythical modular Mac Pro" was completely undefined. Of course, this also turned out to be a one-and-done (may have been some GPU updates?) and was overtaken CPU-wise by regular i9 iMacs.
(Also suggesting the "iMac was to be the new Mac Pro" theory: Apple nerfed the 2014 Mac Mini with a low-powered CPU and graphics, leaving the field clear for iMac to become the only viable medium-to-powerful Mac desktop system. Then - no upgrade/replacement for the Thunderbolt display, although the 2016 LG Ultrafine looks suspiciously like the innards of a new Apple-designed display abandoned and shoved in a plastic enclosure. All-in-one was obviously the preferred route ).
4. 2019 Mac Pro. The "Steampunk" design may be a reference to the late, lamented Cheesegrater but it's really aimed at a different market that wants
multiple high-end GPUs and biblical amounts of memory. Spend $20k and you'll get something competitive with equivalently-specced PC hardware (if you compare like-for-like) - but the $6000 entry level is twice the price of the Trashcan and the base performance worse than a much cheaper iMac - only makes sense if you're going to spend another $10k on GPUs, RAM and storage.
5. 2023 Mac Pro. May look like a 2019 Mac Pro - but it's really just a Studio Ultra with (non-GPU) PCIe slots and a lot of fresh air in a hand-me-down Mac Pro case. There's a market, but its only a subset of the 2019 Mac Pro market. It's already overdue a processor bump and I suspect it's not a product for the ages.