Fountain Pens

So I am more curious than anything. At first I was thinking that a rollerball could right on paper that fountain pens can't but as far as I can tell that is only marginally true and there don't seem to be many "inkball" or "cartridge rollerballs" or what ever desciption they go by. And of those that I have found, they are either plastic disposable looking pens or more expensive pens that leak all over the place. I have read that one of their main weaknesses is that since fountain pen ink is on lower end of viscousity, tolerances must be tight and the rollerball wears much faster than fountain pen nibs. After learning all this I wonder if there is a pen that has addressed all of this. One that lets you use fountain pen, replace the nib when it is worn, and doesn't look just like a disposable pen. Any one know of a pen like that?