I think iOS 11 is an example of Apple realizing that they need to take a step back and focus on the "it just works" aspect of their mission. There are dozens of small fixes that I've wanted for years that they've finally fixed, all in one release. Haven't had the chance to look at High Sierra, but I'm hoping it's the same there.
All of their products used to behave much more synergistically. There was a real feel when you bought an iPod and hooked it up to your iMac that the two popped together. Now, an iPad barely ever needs to even touch a Mac or even PC. The technology is at the point where it ought to be possible to offload display/UI tasks to an iPad - but you can't in an integrated way. They really need to tie their whole product lines together again (through synergistic features).
Not iPad as much as iPhone, but do you use Handoff? I do all the time, between work and home MBP, my phone and watch. For iMessage, web pages, emails, phone calls, etc.
I feel like the only thing we've actually lost was the requirement to use iTunes to set up a new iDevice, and good riddance to that. An iPod really was a peripheral whereas their modern successors are fully-fledged mobile computers.
If you mean use an iPad as an external display, that's a rather niche use. Is there a reason that the 3rd party app Duet Display doesn't meet your needs?
