Strongly disagree. If your system can run it, then it can detect is as well.
Certainly, but it isn't as straight forward as you're implying. You're living in the 90s if you think a virus can't be hidden from a virus scanner.
your malware encrypts itself, there must be a loader program that is not encrypted
Yes, but can that loader or the mechanism for installing that loader not change its own instructions by randomly transposing code? Can it not insert NOOPs here and there? Can it not reassign registers and so on? Is it always going to have the same hash/checksum/bytes?
No, no *good* virus will.
Heuristics are the secret sauce of good AV scanners, but that is certainly far from perfect. Otherwise, virus scanners rely on some form of 'virus signature', which is often based on a database of known viruses and checksums. Nearly any changes at all means you're AV is left guessing... And no algorithm is going to be right all of the time, even *if* the changes to the loader or virus are minimal.