Still, I think this is a great portfolio piece. The designer should keep going, and amass a little collection of simple repairable appliances.
It wouldn't replace the toaster, because that fits on the windowsill and an air fryer would not.
Tiny kitchen here. I'd like to try an air-fryer, but for now my one table-top appliance is a instant pot. It is used daily.
I suppose it's that for some people but everyone I know with one also has a fullsized oven. They are used for different purposes and often you don't want to heat up your whole oven to warm up some fries for 10 minutes.
A toaster does only one thing and takes up space.
An air fryer or toaster oven does lots of things, and also makes toast.
That's why toaster oven makes very little sense for anyone except if they have a "big American kitchen" and want to fill their countertop.
If you have one of those, they (can) replace your toaster.
More houses probably have toasters than all the others combined, but toasters don't really wear out all that often. You buy one for $10 when you setup your household and it lasts a decade or more.
I'm due to buy a new one, because the supposedly decent one I bought less than 5 years ago isn't toasting properly, and either burns the toast or does nothing despite adjusting the dial.
I haven't seen a toaster oven big enough for my family, and we otherwise have an excellent oven with convection features.