Apple isn’t about to go from three to six flagship phones. That’s a lot more SKU’s, and the demand for smaller phones doesn’t seem to be there to justify two 5.4” models. The most I can see happening is four (two sizes in standard and Pro), and I’m not even sure that’s likely, let alone six.
You’re probably right although in the “Plus” era there were plenty of complaints about people having to buy a large phone just to get the premium features that were only available on the Plus variant so it would be nice to cater for this issue. I know quite a few people who are stubbornly sticking with their SEs because they don’t want a big phone but would gladly hand Apple a lot of money for a premium small phone. I suppose it’s more of a wish than a prediction at this point.