Your concerns aren't unfounded, but they're a bit misplaced. Password managers aren't intended to protect you from a local attacker, on your machine, like the malware you described. It is trivial to capture clipboard contents, as you say, but it's also similarly easy to keylog so your passwords would be exposed either way. If you consider your computer compromised or antagonistic like this, don't use it for anything sensitive.
Password managers are mostly intended to help facilitate unique passwords per account, to avoid password re-use which prevents credential stuffing. That is, if an attacker gets a hold of your password from one website they can't use it to log in everywhere.
Back to your concern, there isn't a solution for Windows in this space at the moment. Malware that's alive in your user context (or Satya forbid, SYSTEM) can do quite a bit thanks to Win32 APIs.
You're right, it is done. I worded my comment carefully to leave room for this because it's hard to prove a negative, but I stand by my statement, specificay in refuting the implication from the original comment: we aren't haphazardly blasting plant genome with radiation, at scalr, and guessing it's safe enough to feed to the world. I don't have numbers but GMO crops today are by and large the result of non-radiation genetic engineering.
GMO's are not produced by targeted radiation in the way you have described, at least not as common practice (i.e. the GMO food you buy isn't created this way). GMO crops are generated in two ways: targeted gene modification (with deliberate modifications being made in contrast to the randomness of the radiation method you described) and crossbreeding (which has a more randomized effect but does not involve radiation).
Password managers are mostly intended to help facilitate unique passwords per account, to avoid password re-use which prevents credential stuffing. That is, if an attacker gets a hold of your password from one website they can't use it to log in everywhere.
Back to your concern, there isn't a solution for Windows in this space at the moment. Malware that's alive in your user context (or Satya forbid, SYSTEM) can do quite a bit thanks to Win32 APIs.