Modern toasters are generally way less reliable than older toasters. I think it is very difficult to buy a new toaster today that you can be confident will have a 50-year lifespan.
On the flip side, modern toasters cost $10 and last nearly as long. Not everything needs to last forever, but I've never had a modern toaster wear out. You only think the old ones were reliable because they cost enough that people would pay to have them repaired. That and a huge helping of survivorship bias.
This claim can only be meaningfully examined in like 60 years, minimum, and I have my doubts. I've yet to see a 70s era toaster die, and have personally watched four modern toasters go into a dumpster in the last 25 years. Old toasters were reliable because consumers of the day wouldn't tolerate disposable crap which informed every aspect of material selection and design.
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.
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.
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.
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.