The thing is he has no powers. Even if/when he becomes king the "powers" of that role are purely ceremonial, even if enshrined in law and what paltry bits of a constitution we have in the UK. As another commenter here suggested, if the monarch were to use one of these powers (that they historically agreed not to use) then they'll endanger the privileges afforded to them for merely being the "royal family". They'll happily put up and shut up so they can roll around their estates in their Range Rovers and tweeds accompanied by their close protection unit.

> But maybe if Scotland and Northern Ireland secedes, they'll realize their queen's successor is quite useless.

But they're all quite useless, and many of us already realise this.