That's the problem with signature based scanning ( simple file scanning for example ) all one needs to do is write their own code and payload, if that even ( sometimes as simple as stripping all the comment out of the code )
No one complains that a fork is not really good for eating soup.
Signatures are just one part of the arsenal and should be treated as that. They are not problematic, they are just not suitable for everything.
Heuristic scanning takes it a step further and looks for behavior in a virtual machine
Heuristics and the way of how you obtain information are completely separate. You may use behavioural information, but you may also use anything else, like the structure of the file, the code, etc. Heuristics just describes how you use the information (see also this
article).
What you mean is emulation in a sandbox, which can be and often is combined with heuristics.
(some decompile and examine the code)
Decompilation is done for humans only, computers don't need it. It does not add any information for automatic code examination. So, no, I don't believe that.