This is one of the reasons why I believe an iPad could not replace my laptop yet.
I'm actually trying to decide if an iPad would be enough for me (since I don't need to do pro work with my personal machines).
Given that I don't draw, a (better) MacBook Air is actually better for me, and in a similar price range to a comparable iPad Pro.
A built-in stand, keyboard, trackpad, and a more powerful OS make it a no-brainer.
However my daughter, who is a graphic artist, THRIVES on an iPad. She still has an MBP though.
All this said, I think getting a trackpad for iOS would be nice, but only like sracer said. It needs to be supported like Android does.
I can share my personal experience. I've gone from working and traveling with a 15" MBP, to a 13" MacBook Air, to a 15" rMBP, to now using the 12.9" iPad Pro (2018 version purchased almost exactly a year ago).
I LOVED the MacBook Air, because it was such a departure from the old 15" MBP in both weight savings and speed. But as I got used to the higher resolution screens on my iPhone and iPads, the low-res Air was just too hard to continue working with, so the jump back to a retina MBP was great - not to mention the added screen real estate was nice.
But the weight and amount of space required to carry it and the charger became more than I could handle - plus I really didn't need the horsepower the rMBP had while on the road. So I picked up the iPad Pro last year and have been extremely satisfied since.
One great thing about the iPad Pro, vs any of the MacBooks, is that I can pull it out and start working immediately (I recommend getting the cellular model because of this), without needing to find wifi or connect to my iPhone for service. The negative of the smaller size compared to the 15" rMBP is a non issue, as I can zoom in anytime I need and obviously when holding it you can put it right up to your face if need be.
I did just start using the Pencil with it, specifically for note taking. If I can fully stop writing in my favorite Field Notes journals, the benefit of taking notes in the Notes app on the iPad is that I don't have to try to remember what journal I wrote any specific note in, since I can categorize and search more easily in a single spot.
I'm even finding myself emailing on the iPad even when I'm sitting at my desk in front of the iMac, which I didn't used to do. It's just so easy to use and I kind of like the limited keyboard travel more than the Magic Keyboard.