Finally managed to revive Absynth 5 in my system. Native Instruments dropped it from their active roster, and even those who have purchased it earlier may have hard times installing it on Windows 10.

In my case it was kinda outrage: Native Access (a controlling/licensing/updating software) would download an .ISO file, then report installing it… without actually doing it. No installation path, and version 0.0.0 was the only things I’d been getting earlier this year. Since I did not actually need Absynth at the time, I abandoned the attempts to reinstall it.
And wrongfully blamed Native Instruments for this odd behaviour of the installer.
Then a few days earlier I decided to look into the matter again, as I needed some particularly otherworldly sounds ASAP. The outcome of the effort, I must say, was totally worth it.
Searching the Web, I have discovered that the core problem might’ve been that in my system .ISO files were associated with a particular software, and not Windows Explorer. That was the most probable reason why Native Access failed to install anything. So this kind of files need to be re-associated to Explorer.
Second, as Google’s AI pointed out, there is NativeAccess.xml – a configuration file used by Native Access to track installed and registered Native Instruments products, including Absynth. And it may, in fact, block the installation attempts.
Killing it and relaunching Native Access (which promptly regenerates it) has been called ‘a common troubleshooting step for various product visibility or demo mode issues’.
The file sits here: C:\Program Files\Common Files\Native Instruments\Service Center
On macOS: Macintosh HD > Library > Application Support > Native Instruments > Service Center
Common Uses.
Unfortunately it was not enough. In order to get launched, it was necessary to literally downgrade it to the version from 2013. Thankfully, NI’s site still hosts legacy updaters.
Again, as nerdy as it sounds, it was totally worth the effort – the customer of the tracks I’ve produced with it has just confirmed this. :) Even though I’ve basically used 3rd party presets with only minor altering. The synth’s architecture is a bit too non-trivial.
…I’m very uncomfortable with the idea that Native Instruments’ older synths – FM8, and Massive, – will soon share the Absynth’s fate and become ditched, uninstallable and/or unrunnable on newer systems. The synths themselves may be replaceable to a degree. But not exactly the sounds they are capable of.