I don't have a "Plus" or "Pro Max," but when I moved from iPhone 4 to iPhone 6 I noticed I carried my iPad less frequently when I left the house - the larger iPhone screen was a sufficient improvement that I could read mail and web pages a bit easier.
But I've always had a physical keyboard for my iPads, because when I want to write, really write, a virtual keyboard (or dictation) and small screen just doesn't cut it for me. So iPad will still travel with me when I leave the house when I know I'm likely to have the urge to write or to edit/display my photos on a big screen, or participate in a video conference/call with several people in the room with me, or...
Back home top preference when I want a big screen goes to my 27" iMac. iPad and iPhone are left in the dust. Still, all three are handy and there are times I'll have all three running with different content/tasks at the same time.
In the end, the more tools we have, the more selective and sophisticated we can be about how we use each of them. My electronics tool case has a wide variety of drivers, pliers, cutters, strippers, and other tools... it's just not a one tool world for me.