I disagree with this notion that Civ 5 saw a "dumbing down" or simplification of the series. Sure, Civ 5 vanilla was simple, but so was Civ4 on release. After BNW and years in the modding community, Civ 5 is a rich game, just as much if not more so than 4, and I say this as someone who still has more hours logged in on 4 than 5, and we're talking thousands.
I do agree with TMIT, though, that 5 saw a regression in terms of UI. I haven't really gotten into 6 yet and I don't anticipate doing so soon. But I'm hopeful that after some time and expansions it will prove to be a great game, that's exactly what happened with 5 and, to a lesser extent, 4.
I also understand that perhaps the average player plays on Prince or King, but why does Firaxis never release a competent combat AI? The way Firaxis does it, they make the AI completely incompetent in combat, then they give it massive economic bonuses to present a challenge in brute force for high level play. On the other hand, if the combat AI was actually good, they could easily make the lower difficulty levels more palatable by simply nerfing AI production, so that it had smaller armies. Or am I just dreaming here? To be honest, I just don't know. I know that AI can handle games like checkers or chess on par with the best human players in the world, but perhaps a game like Civ is just too complex. Maybe it's not realistic to ask for a competent combat AI?
I just find it really immersion-breaking when I'm in the middle of a giant war with a peer power and our big armies are squaring off, and the AI just dances its units around pointlessly, basically gifting me the war, allowing me to win battle after battle. There are many times I make a mistake and I think "oh no, my unit's dead", but the AI doesn't kill it. I can play sloppy and still steamroll the AI if my military is even close to being a match.
It also ruins game balance because, on the economic side of things, the AI is nearly impossible to beat on Diety. Good luck sitting there and trying to win by peace. Good luck trying to build any wonders or catch up in tech without any fighting. What happens is that it always boils down to war. Winning by culture or spaceship - without wars of conquest - actually is incredibly difficult to do on Diety. But winning through conquest, or winning by simply smashing your rival's tourism-generating capital, or space ship building capital, is much, much easier.
The easiest way to win from Civ3 to Civ5 was to just start building units, keep building units, and kill, kill, kill, and never stop building units. Screw buildings, screw wonders, screw culture, screw all of it. Just pump out a military and stomp heads. It's really tiresome. Why have anything else in the game if it's not balanced? They really need to improve the combat AI, I mean REALLY need to. The rest of the game just doesn't work without it.