The first run experience of Safari on a brand new — for 2017 — iMac with a spinning hard drive, as documented by “cocobandicoot” on Reddit, is pretty awful. Judging by Michael Tsai’s post, it is far from the only instance of subpar performance on Macs not equipped with solid state drives. A simple solution for Apple would be to treat these Macs as the baseline for good performance; then, everything with an SSD could be much faster, but a spinning hard drive Mac would not be too slow.
However, as much as I favour holding back on the tendency to maximize newly-expanded technical bandwidth, I can think of some pretty clear instances where the much faster speeds of an SSD could make MacOS more capable than would be possible with a spinning hard drive. The system can build caches in the background so something like the photo picker is always current; a document can be saved with every keystroke; software updates can be downloaded in the background — all of these things can happen at once.
It has been eight years since the introduction of the second-generation MacBook Air, which brought solid state storage to typical Mac users for the first time. Apple has been shipping annual updates to MacOS that presuppose the availability of a SSD — APFS, for example, took a year to come to hard drives. And there are plenty of Mac Minis, iMacs, and tower Mac Pros that are supported by Mojave but will perform poorly because they have hard drives.
So, if these features require an SSD — inasmuch as if the Mac in question were not equipped with one, it would suck to use — I don’t think it’s unreasonable to suggest that Apple should not be selling Macs without solid state drives any longer. They’re most of the way there — the only remaining model available with a spinning drive is the iMac. But, even with a Fusion Drive, it’s clearly still not performing to the standard that it ought to be.