I disagree. I found the expansion system to be a big problem in civilization series. They release main game with small set of features, then fix it it with patches. Then they release an expansion barely fitting the original game and try to patch it. Then they release the second expansion with problems fitting the previous 2. Not only the game in this stage is chimera, but game lifecycle is near the end by this stage so we only see 1-2 patches fixing obvious things only.
Take Civ5, for example. The strength of cities in defense makes tall empires too good. With features of BNW which make tall empires so much better it would be reasonable to rebalance the military system to make cities more vulnerable - but this would require changes all over the system and BNW already changed a lot.
Personally I'd prefer all systems in place right away, made for each other right away and later just polish it patches to perfection, no huge additions.
I think you are misunderstanding. Had there been no expansions. Firaxis might have never finished patching vanilla Civ5. A lot of patching work that went into vanilla benefited the 1st expansion.
Games history is littered with unpatched games after publisher pulled plug on the game or due to financial issues. These games usually saw support end early because they never received an expansion and thus, there was no financial sense to keep patching.
My view is, we generally want the vanilla game to be followed up by at least 1 expansion. It gives the developers 1 year plus to patch the base game as they work on the expansion. A lot of that patching may not be possible if the developers decide to cut their losses and just milk the base game sales since they have no budget to do anything more.