> We don't want to complicate the language
So I can understand if this complicates the implementation but I don't know if totally optional pragmas or annotations complicates the language itself. Like C has this but I don't think people say "Ah C is alright but the pragmas are a bit confusing and make things complicated".
> experience shows that programmers are often mistaken as to whether branches are likely or not
Your average programmer may mess that up, but those who would give optimisation hints aren't quite your average programmer. And insisting on introducing PGO to your build process (so build, run-with-profile, rebuild-with-profile) for some cases where someone isn't mistaken as to whether branches are likely (or whether some loops run minimum X times, etc) feels a bit needless.
Please remember though that I'm neither a Go programmer nor contributor so I'm really just an outsider looking in, it could be that this is a total non-issue or is really low-priority.