There's absolutely no reason the iPad can't have both, as in a simple kiosk mode for old people to call someone, and then a "Power-user" toggle in the setting for those who want to enable the more complex features.
My mother in law insists on having the newest iPhone every year. It spends 95% of its time in the bottom of her purse, turned off. 1% of time is phone calls. The other 4% of its time is my MIL complaining about how complex it is while she laboriously dials a number. Realistically, she should just buy a dumb phone[0].
A complex tool like a smart phone or a computer has a duty to provide its user with the capabilities they need, even if providing such capabilities requires a complex UI. You can only make something as user friendly as possible. After a certain point, making something easier to use requires you to remove features.
This is why professional software, like CAD software or DAWs or programming languages, has a leaning curve. It is a tool that you are expected to have to learn to use. If your tool is too complex for your needs, return it and buy a simpler tool. Buying an iPad or iPhone just to make phone calls is like buying a stand mixer with all of the attachments when all you needed was a wooden spoon.
[0] Except that the iPhone is a status symbol now
Nobody said they do need it, it's just the HN trope that Apple devices are "so easy to use" that they'd somehow be better for the elderly, because devices form Windows and Android tend to nag you with more pop-ups and complex questions.
And what if I want to add them? Most likely other users would to if the SW support was there and you could run Mac apps on the iPad once you connected the keyboard and mouse.
Apps made for keyboard and mouse suck on touch&pen devices and vice versa (even if the device is basically able to run both) because most devs pick just one mode of interaction and optimize their UI for it, while the other mode is just “supported” as an afterthought.
That’s why I believe Apple decided to have separate devices for each mode of interaction, each with its own distinct OS&app ecosystem.