Personally don’t see them bothering now tilll intel move off the 14nm process that using/refining.
Not too sure what the 9th gen would bring in as a compelling feature.
Absolutely agree that Apple would find a more compelling scenario with Sunny Cove (Tiger Lake S-Series) with Gen 11 iGPU (Iris Plus 9xx Graphics). When those CPUs will actually appear is an unknown, which may push Apple to use the Core i3-9x00B, Core i5-9x00B and Core i7-9x00B CPUs when and if Intel gets around to introducing them to the public.
Or this is the last hurrah for the Mac mini using an Intel CPU and the next Mac mini will be an ARM desktop CPU variant that Apple is cooking up in their labs right now.