Arga,
I agree with you a communist style run corporation will not all of a sudden wanting to go out and spend big time creating demand. Even while it will create spare time away from their production activities and into PvP. It is their mindset, and that won't change because of my ideas 
.
Players in self-sufficient corporations are clearly different and their structure is less rigid. Now, is it beneficial for them to go open market? I would say yes. That is if they are in the business of PvP and want to do that as much as possible. Any time spend on being self sufficient at item production is time not spend doing PvP. This becomes all the more important with a small player base. The value added per unit of play time for producing low end items is miserable compared to what they are most likely capable of doing the same on the high end. Which means more time doing PvP, which increased overall demand for all the lower tier items as well! People do tend to loose stuff in combat .
What goes for self-sufficient corps also goes for 'other' corps doing PvP, for the same reasons, but they might do other kinds of productive activity to get the NIC required to participate in combat. NeX, obviously ex EVE players that left that game in Rage and left again is hardly a case for either side of the argument here.
The % of those players that were, as you put it diverted, is much too small to have any significant impact on the open market. By which I specifically mean, even if the tax was the SAME or Zero on both internal and open markets, the volume of sales that would migrate off internal markets would be NEGLIGABLE.
Hard to argue against that clear logic that as it is most likely right assuming self-sufficient corps keep being that way by not interacting with the market in any significant way. But negligible is still progress if you ask me for such a tiny change and i would not worry too much about the long term in the current situation. There always will be the case that if existing corporations do not change their mindset, nothing will change whatever the devs decide. My view is that in such a case, we all loose out on more intense activity and thus a more active game!
While soundly put, your case can also be taken and explained differently where the reverse conclusion to yours would be valid too. Currently we clearly have such a thing as idle or slowed down demand. What ain't there or cannot be seen by players can't and will not be bought! My personal experience in this game has been that nearly always when I needed something, it was not in sufficient (or any) volume in corp or out corp via the market interface. Thus no quick and reliable access to needed items. NOT buying and/or completing a bot means NOT fitting it and thus NOT using it and thus guaranteed NOT loosing it. And thus no need to replace it either .
If you measure demand given by current activity, you are missing this demand and then there is the delayed demand due to the time based learning. I would buy seth if I had the extensions for it, but i don't so will not buy it yet . The simple fact is that loosing higher tier bots and items creates more demand then loosing lower tier items and thus generates less activity.
Academics still try to dismiss it (it doesn't fit their currently failing models that well), but there truly is also such a thing as supply side economics. No one wanted a television, until the first one could be bought. Would any of you have missed an iPod if Apple did not bring it on market? I think the answer is a resounding NO! While not a perfect analogy with our in-game situation (we cannot invent new items), it does nicely demonstrate that measuring current sales activity can in fact grossly understate future demand. There was before the TV came out, zero demand for it and thus no one put up buy orders for them in anticipation of its arrival:).
So I think you have to let go of the basis of your argument, which was that future market activity can be based on looking at todays activity and the way corporations now go about their business in terms of market interaction. Things can change and probably need to quite drastically. If it was me I would go further then just the TAX system! Putting everything in one market might be overkill I realize now, but I see no case not to have T1 and T2 be forced public. It certainly has potential to create more activity and a more active environment for new players. But again, if players refuse and fall back in old ways of evading every bit of interaction and at any cost, nothing will help!
I still got two questions for anyone that has an opinion against tax reform (which offers good incentive to outpost mechanics too if your tax there depends on the level of control for example).
The questions I am talking about:
1. Do you have any rationale why a 12% tax is good to start with?
2. Do you have any rationale why using market facilities in a public terminal, for corporation use, should be exempt from tax that is otherwise applied?
