If I have enough control to the point where I can read your memory in some way, I can just use ptrace. Heck, I could attach a debugger. It seems ludicrous to want that level of protection out of a normal program running on Mac/Win/Linux.

Now, if your decryption hardware was an actual separate box, where the user inserts their keys via some mechanism and you can't run any software on it, but simply say "please decrypt this data with key X", then we'd be on to something. It could be just a small SoC which plugs into your USB port.

Or you could have a special crypto machine kept completely unconnected to anything, in a Faraday cage. You take the encrypted data, you enter your key in the machine, you enter the data and you copy the decrypted data back. No chance of keys leaking in any way.