Hm I'd be interested to hear anyone speak to this. When I was on that 6-8 beaver for a while I did notice it wasn't that fun once it got to be knee high and below, which makes sense. Why I'd still be getting use out of my current 9-6 for small daysI might be alone here, but I really don’t like riding my midlength in small waves (e.g. knee to waist as the OP points out). I’ll always grab my groveler, short and wide (5’6”, 22’), or a longboard. I find midlengths to be a lot more fun when it’s at least shoulder+. Nothing beats a longboard when it comes to wave count, my midlength wave count is no better than my groveler.
If you are looking to progress, it’s not just about going smaller. There are so many “advanced” maneuvers to master on a longboard.