A desktop computer cannot be used without peripheral (unless you use it as a server). They where separated from the chassis for flexibility sake, not because they were optional.
There was a time where everything was integrated into the chassis, screen included. Those were hefty beasts, loud, and hard to maintain, because when a peripheral broke, you had to service the whole unit instead of swapping the keyboard to a new one.

As for the Deck, you have everything you need for its intended use, no peripheral needed. Of course, you can add some to make it work like a PC, but in such case, is it still only a Deck ?

We could argue for ages around that, but I think it boils down to philosophy. Some prefers maximalist definition, other prefer minimalist definition. I'm obviously of the later school, and you of the former.
So, how about we agree to have different opinions on the matter, and go on our respective way, instead of throwing oil on the fire of a sterile debate ?