Why don’t they just go back to the older batteries? Or was there a problem with them?
Pride.
They are now boxed into a corner, the entire electronics support package is optimized to that particular lithium technology.
If they revert to NiCad or even a different lithium chemistry they need to redesign all the support circuitry.
Then they need to spend a couple years torturing the batteries and support circuity to make sure they haven't overlooked a new and improved failure mode!
The it gets really bad, they need to make absolutely sure that no one in any third world cesspit repair facility mates the new batteries to the old electronics or vice-versa. Ever, not even once.
Why don’t they just go back to the older batteries? Or was there a problem with them?
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The way I read this, it was a purely mechanical patch. To change battery chemistry (go back to the old batteries) would require electrical system redesign and reverification. That would be way more time, money, and labor intensive than the pure mechanical band-aid.
Probably the machinery that this battery powers is too power-hungry. Safe Nickel/Cadmium/Silver chemistries are far less capable, and there is not enough space to install several of those batteries. (There is barely enough space for one Li-ion battery.) The airplane is heavily electric - it is not cable-operated, as they used to do in 1920s, and it is not hydraulic, as many airplanes are still built today. When you need electric power you cannot pick between operating the left wing vs. the right wing - you need both, otherwise the airplane will flip.
I agree with the opinion that the cause of the fire remains unknown. Most likely it is a singular manufacturing defect. Those batteries are not built as accurately as an integrated circuit; they are just plastic bags with metal foil and assorted chemicals, roughly rolled together. There is no way to assure repeatability; and QA testing is not likely to discover a short that hasn't happened yet.
However if the failure of the battery is no longer a danger then what can I say? Airplanes are not something that I use these days. (I'm still waiting for TSA to be disbanded.)