I have to respectfully disagree. Over the last few seasons I have experimented foiling in higher winds with five, three, single, strutless and Peak4s. In sizes 9,8.5,8,7,6.25,6,5,4,3.5 and 3m.
I remember being stoked holding a 9m 5 strut Rally into 20 knots and feeling like I'd found something completely new! Eventually, with more experience and skill, you realize the same advantages that make the lighter kites a better option in light winds are at play in winds over 20 knots. The speeds at which you ride down wind/swell are fast and absolutely bring weight front and centre to performance and freedom of movement.
You are correct that above 20, everything will work, but extra struts and extra weight don't make a kite turn faster. Smaller kites turn faster! Struts give more solid structure on the high end, but they have diminishing returns as the canopy size starts to get below 6m. For those truly unwilling to drop the struts, a single strut is more than enough in kites 5m and down. Still, they drift, recover and purposefully stall back nothing like even lighter and smaller kites. The difference in dexterity and depower makes for a pretty different overall experience. Like many venturing into higher winds on foil, I spent a fair bit of time overpowered on three strut 6m kites. Eventually you find yourself in waves you are willing and able to take advantage of on foil only to realize your kite won't let you go and do what you want. The moment you try, you have way too much power. You can safely go out there in among these phat tantalizing swells, but can't actually ride any. This is a truly agonizing moment and generally once is enough to get you shopping for something that will let you ride the way you want. Sizing down in both weight and canopy opens the options right back up to where they were in less wind and you realize that three strut kites really have no place in high wind foiling. They can be used just as any kite "CAN" be used but eventually, you won't bother. You will either go back to surfboards an miss out, or buy in and get a 3m.