This is a really poor mechanic, along with the diplo victory changes it's a huge step back from previous civ games.
The simple fact is that it will be possible to blitz and take multiple capitals very quickly for a cheesy win, or the game is broken in other ways.
If it's not possible to blitz and take at least three enemy capitals on the coast, usually enough to win an endgame in other words, then naval invasions are probably close to impossible anyway, which also sucks. I do think this may actually be the likely outcome - that invading from the sea/another continent will be just about impossible against the AI at a certain equitable level, and of course human players will be able to defend flawlessly aganst otherwise far more developed foes. So maybe a step backward, navies were poor enough in previous games and they got worse, I could accept that, but it's not really what everyone is hoping for I think.
The other possibility, again mentioned before, is that the AI have hardcoded some really weird obsessions about their capitals, specifically for the purpose of making conquest victory harder. This works for that idea, except it'll lead to ridiculousness in normal gameplay if a player can just take many of their other cities and cripple their empire exploiting the AI's obsession over its capital.
All these scenarios do involve some tactics and skills from the players - coordination, feints against the capital or whatnot, but in the end it still really lead to cheesy wins. There just wasn't anything wrong with the system before that they had to change it, and most probably agree it didn't work in CivRev.
And who knows, Diplo victory might be even worse in the end, after all complaining about tedium in combat/conquest victory is probably nothing to the complaints of a random player who lost a game with a huge empire, tech lead etc... and another AI was friends with more city states (since Diplo victory appears to depend not at all on population, empire size or other factors)