Unfortunately, this is really not Apple's issue but Intel's one. For some reason, Intel chose to configure these CPUs this way. Since it is essentially the same CPU core (just with different clocks), the max clock they can hit in sustained scenarios will be comparable. The only real "solution" to this I can see would be limiting max turbo of lower-tier CPUs under load, which would not necessarily be customer-friendly. Interestingly, prior to Coffe Lake Intel had more detailed differentiation of max turbo clocks that CPUs could hit under certain scenarios — this helped to differentiate them more; but it seems that they have changed it with Coffee Lake. A funny thing is that usually CPUs are binned so that parts with better power-performance curve end up as more expensive models, but so far, there doesn't seem to be a difference between an i7 and i9 — both reach similar clocks under similar power load.
And sure, Apple could also try improving the cooling solution so that they can dissipate say, 60-70 watt instead of the current 45-50 watt, but I don't think that this will really solve the CPU similarity issue — they will still perform similar to each other, since all of them would be able to get higher boost. And finally, if you want to take full advantage of the i9's potential, you need a desktop cooling solution since they get hotter than an average desktop CPU 🙂 Its a strange SKU indeed, and it shows how Intel is desperate to sell stuff. Anyway, the i9 does offer a healthy boost in short workloads, so it's a good choice for people like me who primarily work with short CPU-intensive workflows.