For people who view camera settings as an inconvenience, then yes that sounds like an attractive thing. When using a camera as a tool, a photographer sometimes wants complete control over the capturing process. Some things you don't get to change later no matter what algorithms are involved, such as the balance between noise and exposure time.

If the camera had hardware light metre, then recorded the ambient light level as well, then wouldn't it be more powerful to be able to make full exposure adjustments after the fact?

No because the act of taking the photo freezes the exposure settings due to how photos work.

I've played a bit with astronomical photography and the amount of work to getting the signal in the picture good enough without introducing noise is quite high. (Going as far as taking pictures with the lens on the telescope to capture the natural sensor noise, which is usually good for about a few hours of use)

However, you don't get second tries on a computer. If your signal is too weak or the noise is too strong, the picture is ruined, no amount of editing will fix that.

The only solution I'd see for that is to take multiple images at once with different sensors, each using a different exposure. But that still doesn't provide a sliding scale, only a stair function of "is this close enough"?

The better solution is to take a proper light meter and properly setup the camera in the first place.

Or you can shot a several shots with different exposure simultaneously (see e. g. Light L16). It may not work for an astronomy but should work in many other cases.

That's not really a solution, especially because that requires a lens per exposure setting (you can't expose a sensor with different settings at the same time, that's basic physics). So any number of exposure levels over 2 is going to be a huge problem.

An unsynchronised manual gearbox also gives you full control over the process. And like an unsynchronised manual gearbox, a fully manual camera is great in a few situations but most of us don't find it that convenient as a daily driver.