Any design update comes with cost involved (development, testing, tooling, spare parts, technician training etc.), so the age alone imho is not a sufficient reason to update just for the sake of it.
A (mandatory) technical reason for a form factor change is not obvious to me as well, as the current case offers enough room for relevant ports and cooling.
That leaves marketing as sole driver for a design change. In that case, you could argue that Apple actually changed the design only 4 years ago, when they introduced Space Grey as case color for the technically significantly changed 2018 minis.
They could have changed the form factor back then and even could have argued with technical necessities. They choose to not do that, but instead just updated the proven form factor, but to keep the minimalist design. So why should they invest a huge amount of money to change the form factor for one of their lower-priority machines, if there’s no requirement to do so?
Unless - of course - there are technical changes that are yet unknown to the rumor mills, such as some kind of new modular system to be introduced, where you could plug-and-play additional modules with e.g. an additional CPU node, a storage container, a Raid box etc. onto a base unit.
But if they would indeed plan something like this, I would expect them to use the Mac Pro (Cube) platform for this and not the more price-sensitive mini platform.
So to me a form factor change sounds more like wishful thinking on your part - sorry to say that! I’m just failing to see the real necessity for such a change that could convince a rational guy like Tim Cook to approve significant spending on a proven platform that could easily house the next generation of electronics in its current form. Guess we’ll see in a couple of weeks