I've had an iPad since the first generation and I loved the iPad. A year and a half ago, when the air and the mini retina came out I agonized for a while trying to decide whether to replace my iPad 3 with the air or the mini retina. I finally decided to go with the mini retina and I loved it. It was so much more portable than the full sized iPad and I used it much more than I used my iPad 3. I also brought it with me much more often as well. I have a Windows laptop, but at this point I rarely used the laptop... I did almost everything on the iPad or the iPhone. I never considered getting rid of the laptop as there are some things that can't be done on the iPad, but would easily go a week or two or even longer without picking up the laptop.
Last fall I got the iPhone 6 on launch day and immediately my iPad usage dropped. I had be using it daily, but after I got the 6 I would go days or even a week without picking up the iPad. I think part of it was 'new device fever' and over time I started using the iPad a bit more again, but still not nearly as much as I did before I got the 6. I could still go a couple of days without touching the iPad.
After a couple of months I started drooling over the iPhone 6 plus in the stores and I wondered if I had made the wrong decision getting a 6. A bit more than a month ago I finally gave in and bought a 6 plus. It took a little time to get used to the size, but I love the bigger phone. After using it for a couple of weeks I sold my 6 and I haven't looked back yet.
Since I got the 6 plus my iPad usage has dropped even more. Again I'm back to picking it up only once or twice a week. The funny thing is that my laptop usage has actually increased since I got the 6 plus. Now it seems that sometimes I just want an even bigger screen than the 6 plus or the iPad mini, so if I don't want to do something on the phone I'll pick up the laptop instead.
At this point I'm not quite sure what I want to do with the iPad mini. I do still prefer to use it occasionally (it's nice for watching movies on a plane, for example, but I only do that a few times a year), but for the most part my phone or laptop are fine. I'm thinking I'll probably sell the mini, but I'm not sure if I'll replace it with a bigger iPad or not. Like a PP, I think I may wait to see how the iPad pro looks when (or if) that comes out, but frankly, unless it has more 'laptop like' features than the current full size iPad I don't think I'd get it just for the bigger screen. When I was using a full size iPad before I didn't always take it with me because of it's size... and adding a couple of inches to the screen size won't increase the portability.
So, if anyone is still reading after all that, I think the answer to the question is... yes, it's definitely possible to get by with the iPhone 6 plus without having an iPad, but there are times where I do want to have a bigger screen. A laptop or an iPad might be good for those times.