I am not sure if it is a lot of people or some people and what is the degree exactly. I mean we obviously cannot know for sure here is how I see things:
For people that bought the 12.9 inch iPad Pro because this is their only computing device and they want to use it to replace their laptop the proper external monitor support might mean that they would not longer rely on the iPad screen for the majority of the work as they can hook up big monitor. If this is the case they will then evaluate how much they need 12.9 screen real estate for the pure tablet work (like pencil work, media consumption in bed, portability etc) and they might decide that 11 inch kind of does the job too.
Now we don't know what is the percentage of people that considered the 12.9 inch screen because of the potential laptop/computer replacement and how much people want the screen real estate for tablet work. So there would be some division for sure. How much no idea.
On my side I could tell you that I am not able to use 12.9 inch size as a tablet. My hands are small, my wrists are weak, I would not find it comfortable at all. On the other hand if I did not have any other computer using 11 inch screen would not be healthy for my eyes. I would need at least 12.9 inch even 14 inch to be honest. So I assume this is the kind of group that would reconsider 12.9 inch iPP if there was proper monitor support.
ETA: Now that I think about it maybe this group was already filtered with the ability to duplicate the screen. So I take my words back. Sorry, going in circles. It is not easy to be me
. Anyway what I want to say is that if you plan to duplicate the iPad screen for the most time and use the bigger monitor than the iPad size is relevant only for the tablet usage. If you plan to extend the screen then 12.9 actually makes more sense because I think that it would be weird for people to switch between 11 inch monitor and say 27 inch monitor you know. I don't think that our eyes would accommodate to that.