It’s not what you ask for, but one thing you can buy is diamond wire used for cutting large blocks of stone. For example https://www.dexpan.com/collections/diamond-wire has 100ft wires, and they sell a hydraulic crimper. To me that suggests it is possible to connect two of these to get that 50m length.

That is a lot thicker than what you ask for (about 1 cm², compared to your 0.0012 cm², so about 800 times as thick) and more flexible.

Unless you’re cutting a very weak material, I would think that thickness is as good as unavoidable. If you pull at one end of a 50m saw, and, say, 25m of it grips the thing being cut, would the fairly small pin through the end of the blade that you pull on be strong enough to not break? Or are you going to move the blade very slowly through the thing being cut? Alternatively, maybe you want fewer teeth, to decrease the maximum force needed to pull this blade through whatever you’re trying to cut?