A new OS isn't so easy to do. AmigaOS is a good base because you have something that already is quite usable and can be improved well.
Why would Germany buy something when it's available for free? There is BSD, there is Linux, and there is AROS. All three run on cheap x86 hardware and AROS will eventually be fully ARM native (right now it's just runs hosted on ARM Linux). Why would Germany or any other country want to chain itself to a dead end arch and pay for a OS that is decade or more behind modern OSs?
If the CPU cache strategy distinguishs between program code and data, less code can make a difference even if you process gigabytes of data.
Porting a big application (let's take some relatively easy but big port, such as Firefox for example) to AmigaOS doesn't automatically reduce its amount of code...
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A new OS isn't so easy to do. AmigaOS is a good base because you have something that already is quite usable and can be improved well.
Sounds like you seriously underestimate the amount of work needed for "making AmigaOS competitive", and also overestimate amount of work needed for new OS, compared to "totally redesigning existing one"