Why have a commuter car and a big pickup truck? Because they serve functions at completely opposite ends of the scale. Going for one device in the middle will compromise both functions. The more critical those opposite functions are to you, the less it makes sense to compromise with one device. Of course as with all investments, finances are a factor to consider.
For me, I need the large 12.9 ipp for work related tasks. I don’t want to compromise on work so I’m not willing to go smaller. And I like to do a lot of reading so for me it’s worth having the small, comfortable iPad mini in addition. If I had to read on a bigger device, I would end up not reading nearly as much (I tried that), so I’m not willing to go bigger.
Not to mention the mini was only $150 after a discount and trade in. And on top of that I don't upgrade my devices until I absolutely have to. I imagine I’ll have these iPads for another 4 or 5 years probably. Compare that to people who upgrade their single flagship iPad every one or two years at full price minus a trade in maybe—I’ll have spent much less.
It doesn’t make sense for everyone to have two different iPads, but for some it does.
Because the pickup truck and commuter vehicle are completely different beast in the sense that a pickup truck can perform duties that a commuter vehicle can't without additional assistance, possibly. A pickup truck has the ability to load an 85 inch television set, possibly haul a pair of motorcycles, etc. along with blocking the view of other drivers. A standard commuter "cannot."
The ipad 12.9/10.5 can do (should be able to) the same exact thing as the mini .... unless I'm mistaken. Someone list differences, please. Not hardware specifications, but task that can be accomplished on a tablet.
I'll add this however, I'm referencing individuals carrying both a 12.9 and a 10.5 (and even a 9.7), both of which are essentially the same so to speak. Yes, there's a size difference comparatively but when lumping the mini into the mix, we can say the 12.9/10.5/9.7 are basically the same size ... larger ipads, and the mini ... smaller ipad. I can theoretically see having a larger and a smaller tablet, but having two larger ipads seems ... I don't know....
Side remark: Personal experience, even though I have 20/20 vision, I know reading on the mini is not comfortable at all because of the need to scroll side to side, etc. Not the same as an android or iphone, but reading on phone offers the same personal experience as the mini. It's simply not a comfortable ... personally.
Used to have a laptop and an ipad. Now I have a big ipad and a little ipad.

The big one is my main one. The little one is for when i want to look at documents while working on the big one or for having something to throw into my purse. The main reason I want to upgrade it is that I’d really like ASK and pencil support available at all times, including on those quick jaunts. Also, if I’m only going to work a little on a trip, I’d leave the 12.9 at home and opt for a 10.5.
If I were in your shoes, I would simply have stuck with the 10.5 seeing as how it probably gets more usage. I personally only have one ipad for everything because daily usage, everywhere simply defaults me to using my 12.9 ... always. I have don't have the itch to grab the ipad 4 retina when I'm out and about. If I need to use an ipad anywhere and everywhere, especially for portability, I simply grab my 12.9.
After getting my ipad 12.9 2017 late last year around November or so, my ipad 4 retina has essentially not seen the light of day ever again. I guess that's why I don't quite get the point of having two ipad especially if they are of similar sizes. Yes, I've read individuals' reasonings for having both.
I’m gonna need people to quit with the positive comments about the portability of the 10.5. ;-p
Every time I talk myself into keeping my 12.9 and NOT downsizing, I come back here and think about how practical it would be to have a more portable iPad.
I would do it in a flash if I wasn’t worried I would miss my big screen. The 10.5 is such a great size for throwing in my purse and taking with me, but the 12.9 is great for using at my desk. I wish I could have both, but since I HAVE to have LTE (it’s my only internet), I’m not just looking at cost of pad; I’d have to add another dang monthly access fee too.
My suggestion before jumping ship. Seriously give multitasking split screen a good thorough test run on your 12.9. You'll be thankfully for the added real estate.