You're not wrong.
Apple's achieved having a rather powerful, networked, lightweight & easy-to-use computer
that, miracle of miracles, you can carry around right in your back pocket!
And her name is "iPhone".
Other than making these devices less expensive/more powerful,
as time goes on, what new thing is there to make?
You already have the pocket computer: so now what?
"Apple Watch", you say? Fine: an iPhone on your wrist.
But after a portable, powerful computer, what thing is there beyond that?
Like you say: we've hit the plateau & it's gonna be that way for a long, long while.
The only'somewhat slim hope I have is thinking that Apple's not building
this huge new campus based solely on anticipated future iPhone sales.