The point is that although it works, it's fairly difficult and very impractical for most users (need to get flasher, soldering kit, etc. just for downgrading). I imagine that downgrading 3DS may be similarly difficult, if not more. As such, people may be more willing to get a new v4.5 3DS instead of fiddling with the hardware.
I think it's also worthwhile to consider the fact that 3DS with firmware below 4.5 is a "finite resource". Sure, there's still a lot of them circulating around the market, since any 3DS made before 2013 should be 4.5. However, that won't last forever. Eventually 3DS with 4.5 firmware will become virtually impossible to find. If 3DS still cannot be downgraded by then... well, then a lot of people will just be screwed.
I think it's also worthwhile to consider the fact that 3DS with firmware below 4.5 is a "finite resource". Sure, there's still a lot of them circulating around the market, since any 3DS made before 2013 should be 4.5. However, that won't last forever. Eventually 3DS with 4.5 firmware will become virtually impossible to find. If 3DS still cannot be downgraded by then... well, then a lot of people will just be screwed.