This idea has some merit.
However, it is not enough reason to have many kinds of OS.
It is very possible to put up resistance to a virus with just one OS. The one OS can have several 'strains", not mutations, rather variations in the low-level structure which would make it hard for a virus to identify the build of the OS.
Windows, as well as other OS, already hae DLL programs. This can be extended to that a hostile program can not 'crack' the DLL,
err...
a DLL is an In-process library. It's loaded in the same process as the caller. malware doesn't change this.
What malware does is install a global hook on certain DLL functions, for example, they usually hook CreateFile and other file access functions to hide files. AV programs do this too to detect when a f ile is being opened.
There is no "weak spot" in a dll. it's in process; you load it, and it belongs to your process. the DLL allocates data in <your> address space. it uses <your> stack, it uses <your> heap. It uses <your> process memory so you can do what you please with it, including changing jumps and ordinals.
Most "hook" malware is written via a early-loaded DLL, often a winlogon notify hook. this sets it up so that every process will be implicitly bound to a malware-designed DLL, which, when it's DllMain() is called, can hook the functions that would otherwise go to kernel32 or user32 or whatever and do what it pleases with them instead.
rather variations in the low-level structure which would make it hard for a virus to identify the build of the OS.
That's ridiculous. the very same "variations" and randomness that would make malware writing difficult also makes diagnostic utilities and process analysis difficult. What we need is a better heuristic that can better detect malicious use of those functions, not obscurity of those functions.
it a proper program would have no problem with the DILL. The hostile code wants to find a weak sport inside the DLL. The friendly program only wants to use the DLL, not pick it apart.
Microsoft has already hinted they may do something that that in the future. Thus you will have hundreds of different 'builds' for just one release of Windows, All the API is the same. Just he binaries vary in some detail. Enough to make it very had for a virus to attach itself to a bit of code without bringing the system down.
Is this off topic? Well, I am trying to say that just as you can have many flavors of Linux, that can also be done with Windows without really making any visible changes. Which is even better.
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