Not really referencing this thread in particular but there's a backlog of 12.9 owners lined up to defend their purchase against the plucky upstart that (in some ways) comes across as fancier. It'd be the same if the two iPad sizes were released in the other order.
I suspect the sales will clearly show what size most people would prefer.

I genuinely don't think most iPad buyers are after a laptop replacement/device that is best used on a stand/table. The 12.9 is an incredible device for those who want/need it and it's no surprise it has a loyal following, especially somewhere like MR. But it's still a very
weird device.
How big is too big? If Apple released a 15" version I wonder how many of the 12.9 people would call it too big, despite being able to use the same points in favour of its bigness. Why not even go 17"? MacBook Pros used to and they fitted in bags. They were going to be used on tables anyway! ... I'd totally buy a 4K 17" iPad. @_@