Comparing what is possible now with what was possible before is no valid argument
Those high ISOs can serve wildlife filmers. But they can also bring power consumption for lights down. Most of all: closing down iris a bit in darkshots gives you control over DOF (and focus), while it used to be all about fast lenses with open iris. (Although you don't need ISO 409,600 for that. That can overexpose cloudy nightskies
I'm actually surprised I haven't seen any cops with A7s yet: it is seeing in the dark with color
)
Exactly. There was a time when film stocks with ASAs of 200 were new. I'm sure there were plenty of people at the time wondering what on earth you would need 200 speed film stock for?!
Better high ISO performance means you can get away with less lights, smaller lights, lower powered lights etc. Means less heat and sweat for the set itself.
Imagine a scene where you need a large light to act as a moonlight, for example. The difference between shooting at 200 and 1600 ISO can mean the difference between a 12k HMI and the generator/power distro to run that, versus running a 4k HMI. Now, there are plenty of reasons why you should go for the 12k anyway (or a different setup altogether), but the point remains that cleaner ISOs aren't necessarily a bad thing, and can co-exist with lighting your scenes appropriately and effectively.
As for interchangeable sensors - for the most part, the delicate electronics surrounding the sensor and the sensor itself all contribute to making the image look the way it does.
Not only that, sensors are
sensitive pieces of electronics, which is why sensor upgrades are usually done by manufacturers in dust-free rooms and qualified technicians. Film is easy because it's purely and entirely mechanical, and the 'sensor' is each frame of film.
The vaporware Craft camera has, in a sense, an interchangeable video element - although that's more of a modular design than an interchangeable sensor in reality.
Look at it this way - when you can use all the same accessories on different RED cameras, they essentially have interchangeable sensors - it's just their 'sensor' module is what they call the brain.
Aaton tried to do an 'interchangeable sensor' with their Penelope Delta, where in theory you could swap between a digital back and a film back. It didn't eventuate - I think the realities of attempting to do so prove to be technically very difficult, increases the price of the camera to a point where it would struggle to stay competitive and turn a profit, and eats into profit margins on potential future cameras.
