I think the iPad is an amazing device which for some can replace their computer needs but that definitively doesn’t apply for everyone.
I do music composition and production as well as graphic design, and that cannot be done on an iPad. MacOS is just more capable and runs more apps and has more customisation etc. for instance importing wav files into iTunes is not possible on an iPad, which is a small thing. I use mainly Logic Pro for music and the full Adobe apps for design And these apps don’t exist on the iPad (Adobe does but it’s not the same as desktop). The ram limitations are another aspect, because using samples for composition needs a lot of ram.
I also have a MBA (2015) which I use when I write text or researching stuff when I’m out of the house. i also have an iPad Air which I love and besides the use cases I described for the iMac and the MBA, the iPad is my go to device because it’s so fast easy to use and just great. Browsing, messaging, facetime, researching, emailing, some music listening and streaming. Annotating documents and research papers on the iPad was a game changer for me as I would be di this easily with the pencil on the train so that when I arrive I do writing work immediately on the MBA thanks to what I did on the iPad.
Different devices serve different purposes and it all works well together.
Additionally, when I’m home, the iPad has replaced my iPhone basically. I take all calls on it, all messages etc and the phone just lies somewhere and relays calls etc to my watch and iPad.