Well, C already "adds a lot of value". For example, the mere act of defaulting to a calling convention, is already a commitment. The default generated, will not

Well, C already "adds a lot of value". For example, the mere act of defaulting to a calling convention, is already a commitment. The default generated, will not always be locally optimal. No matter how much the compiler rethinks its default choices ("optimization"), you will still end up with sequences of assembly instructions that will be considered stupid on closer inspection. Hence, you already unavoidably pay a price there. In the real world, nothing comes for free!