It's more like to knock out trade for your entire empire, all they need is to blockade your capitol, and kill a road or two. Those two cities on the same island half the globe away all of a sudden have 0 trade despite having a road or port connection.
Imagine that trade is anywhere near the level of your economy as Civ4. Open up a Civ game on Emperor+ difficulty with a large empire, and see how much gold all your trade routes give you. If all that gold went away, in some cases you'd have 0% science. I'm willing to go as far as to say that if I wanted to kill a com with 10+ cities on a map, and I had a chance to take his capitol, I wouldn't. I'd simply blockade it and kill the roads and watch him die. And if they don't just fall over in a few tens of turns with no trade, then I'm arguing that trade isn't valuable enough.
It's so... binary. It's stupid. I get that capitols are supposed to be a big deal, but this is too much. What if we made a rule that said "You lose the game if your capitol is blockaded"? Both of the arguments I quoted would still apply.