Iris Graphics integrated graphics were withdrawn from Intel desktop CPUs with Skylake CPUs and Apple never even used them and they are already discontinued.
Raj Koduri has
joined Intel from AMD and we may see better iGPUs in Intel CPUs in due course but not for the 9th generation Intel CPUs linked so far. For the last few generations we've had broadly the same graphics - variations on the HD630 that's been around since Kaby Lake CPUs.
In the meantime, better iGPUs are only applied to mobile CPUs at the moment and Apple seem to be moving away from those too given that the 2016 MacBook Pros only use Iris Graphics on the 28w CPUs as used in the 13" MacBook Pro.
These are primarily mobile CPUs and can't match the horsepower shown by the desktop CPUs that Apple used but most importantly performance per $ is no contest when they are compared to the 65w CPUs in the 2018 Mini.
So, yes, it might have been nice to see Apple offer a version of the Mini with discrete graphics but that was never going to happen in the existing case which already has an improved cooling system to deal with 65w of cooling in a case that previously was in difficulty with 45w from the 2012 model when it was caned.
My thinking here is that reusing the existing slot-in form factor is really useful for the server farms that would buy the 2018 Mini in the hundreds or thousands of units because they have existing racks designed for the Mini - and I have no problem with that if their input persuaded Apple to update the Mini in the first place. There really is no need to berate Apple for apparently looking like they lack imagination with the current Mini because there's a Modular Mac Pro coming next year.
It would make sense for the 2019 Pro to have everything inside the same box (specifically room for a GPU card) and have CPU benchmarks far in excess of the i7 option in the 2018 Mini.
For this reason I think we might find the 2019 mMP to start far beyond the budget of an enthusiastic would-be Mac mini buyer who isn't keen on eGPU.
And finally, now that the Mini is using desktop CPUs I don't think there will be any appreciable improvement year on year to make Apple want to know about CPU refreshes until at least the process is shrunk from 14NM to 10NM. There's no point getting speed bumped models year on year for the Mini, especially with its track record over the last 6 years given what's been overlooked.