Didnt completely follow the last bit of conversation here, so sorry if i misread / misunderstand something, but from what i seen, the 1.4 had pretty decent Image Quality from rather Early on, and while it is still by far not a bad camera in any way, i would think that most people would mainly use the 1.4 for Higher framerates or general Highspeed Use. Anyone serious about High End Image Quality would want to use an 2.1, as even the 1080p FullHD Resolution the 2.1 Shoots is pretty much the Lowest Resolution that is still considered worth shooting for Entertainment/ Advertising/ ect. (4k is by now about Standard in mid to High end Consumer Cameras, 6K and 8K are coming... I personally still use mostly 1080p anyways, and Shoot 4K casually) today, with 720p HD being barely accepted as "high Quality" still. That in turn means that anyone using an 2.1 probably also is way more serious about getting High Quality Footage out of there. Given that there is also way less complaints about the 1.4 Image Quality to be heard, i guess the Focus of Development has become the 2.1 Image Quality. On the 1.4, as already has been said, you wouldnt really have much reason to upgrade anyways, if you just want to get Highspeed Footage of [whatever], and i assume most 1.4 Users wouldnt Upgrade camera Software regularly, but on the 2.1 there was also the Issue with the first batch of Sensors not being quite Perfect, so they behave Weird in some Situations and such. Causing Lines, Reduced Dynamic Range, Blacks Randomly cutting out completely in some Areas of the 2.1 Sensors and a lot of other things.
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A good bit of all those Image Quality Improvements are still workarounds / fixes for those Problems that caused, not sure how the Current Batch of Sensors is Quality wise, but the first batch, of which i got one, was fairly bad. By now (from what i can tell so far from my rather limited amount of testing i did until now), with 6.0 and 7.0 Software, these Problems of the 2.1 Sensors have mostly been fixed, and you can get quite good footage from them in most Situations.
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I dont own an 1.4 or have access to one, so i cant tell how these Improvements would affect one, but for the 2.1 these Fixes/ changes/ Improvements where Absolutely needed. Still not perfect, but way better than 5.1 and anything pre 5.1 (for most settings at least. Low Framerate stuff on the 2.1 is still best on Build 0.5.037.). With current versions, i can at least shoot Something without some Insane Lighting Setup and still needing to run heavy Denoising and other extreme Processing to get Presentable Footage.
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Otherwise, anyone owning or using an 1.4 can and should still Post about any Issues here and i am sure they will be fixed sooner or later, but the 2.1 Sensor just seems to have an order of magnitude more Problems In terms of Image Quality, then the 1.4 Sensor ever had.