The real world works exactly like that. It's called
System Requirements. If you want to use a piece of software, you look at the requirements and if you don't meet the requirements then you can't install it.
All websites have system requirements, no matter how non-libertarian they seek to be, for example most websites will not work on IE6.
It is not a fantasy to inform users that they don't meet the requirements, it is actually the courteous thing to do. If more websites were bold enough to inform users of their outdatedness then we would not be having this conversation.