The MMORPG example is completely different.
A Massively Multiplayer Online Role Playing Game has a different fan base, game design, and money-making scheme. The objective in an MMO is to have a ton of people always playing at the same time, in the same world. And they don't all play "together". But having them there is part of the atmosphere.
Civilization is not a massively multiplayer game and never will be. That would be a different game, with an entirely different feel. Only people that play "together" ever see each other, so it is ok to "split" the multiplayer community along lines they have already split themselves: those who like the game enough to buy an expansion, and those who don't. And if for some reason anyone ever wants to cross that boundary, they can make an exception and have the person with the expansion play without it for a game. Unlike an MMO, you don't need to constantly have those features active.
In an MMO, you are supposed to have 1 persistent world that everybody plays in. And you need as many players as possible to play at the same time because they all affect each other. So when you make an expansion, you can't leave the audience that only sorta likes your game behind. You need that group to keep the massively multiplayer feel alive for the rest. And vice versa, you will lose all of your casual players if the hardcore ones leave for the expansion and depopulate the servers. So you have to keep both playing with each other.
As for putting things into a patch, I think that is a terrible idea. It is not the same as an MMO.
1.) In your example, they patched the mechanics in, but none of the content that uses those mechanics right? You had to buy the cool dungeons, races, and classes. Other people could play with those classes and affect your world. This is because it is an MMO as I have said, and you need to keep everyone together affecting everyone. In order for the game to not break when a player with the expansion content interacts with a vanilla player, you need to patch the mechanics in for everyone! Just don't let the vanilla players use them.
2.) As a replayable game with semi-randomized components, there is no hard-coded content like in an MMO. You can't give the mechanics in a patch and guard the content, because the mechanics ARE the content. The whole point of the expansion is to give you new mechanics to play with. The content is generated by the game from these parts. Civs are mechanics too. The only part that is "content" that you can hold from the player are the AI leader screens.
3.) Your MMO analogy in relation to Civ would be like if they gave everyone the new civs, religion system, espionage system, melee ships, Great Admirals, new wonders, new units, and new techs, but only the AI can use the content. You get to see how cool it is, but you never get to use it! You never get to play with it. You are playing an entirely different game.