Firstly, I disagree with your characterisation of a reduction in constraints in supply as a 'difference' in constraints.
I also note that my initial reference was to 'capacity constraints on production'. I take it from your silence on capacity constraints on production and limitation to demand-side ceilings that you concede there are no constraints on how many items an NPC vendor can produce nor how fast nor are they dependent on materials of any kind sourced from any place.
In fact, every single in-game supply-side constraint is absent from NeX items. Thus, your argument by analogy with the introduction of different ship types manufacturable in game by player-characters (we have Machs, people still fly 'pests) is misleading: the price and availability of faction and T3 ships is not only limited by the number of people who want them and are willing to cash in PLEX (or forgo buying a PLEX to save themselves sub money) for them: their supply is limited by additional factors not applicable to T1 ships (you don't have to take any special means to obtain a Tempest BPO, while you do a Mach BPC, and you have to rinse-repeat for continued manufacture; as for T2 and T3, they require special skills and special materials not needed by T1). Not only will NeX items not have supply-side constraints higher than existing items: they will not have any at all.
Secondly, as I said in my first post, as someone who enjoys manufacturing and market PvP, the introduction of a competitor into every single station, if they are selling comparable or better goods to me, who has no supply-side constraints, would be disastrous for my enjoyment of the game. Any market niche occupied by NPC vendors is not one players can participate in to any profit, as they have zero costs, infinite immediate supply, and an arbitrary price unrelated to mineral or other component prices.
The outcome on the economy of Eve would be for those sectors of it to be vacated by players. This would reduce demand for the minerals and components used in those sectors, resulting in players also vacating the supply-side activities involved in supplying those minerals and components, i.e. mining.
Now, this may not seem to you to be a bad thing. Fewer industrialists and miners, in exchange for the convenience of an NPC vendor able to provide you with a ship instantly in every station, perhaps a fair trade for some.
But for those of us for whom those activities are our gameplay, the introduction of magiced-out-of-nothing ships or modules would remove our playstyle from Eve.
And I believe that reducing the diversity of activities and players is bad for Eve overall.