It is a possible attack though, isn't it? Even with a fully encrypted disk with LUKS, someone could replace your boot manager with a tool to capture your encry

It is a possible attack though, isn't it? Even with a fully encrypted disk with LUKS, someone could replace your boot manager with a tool to capture your encryption key. For most people this will be a non issue: the cost of such an attack would highly surpass the benefits. But I suspect some people would be valuable targets for this.