It's easy to say, but hard to hold up this kind of statement.
Well, the only thing really needed to "hold up this kind of statement" is to play a game of SMAC and have fun, and check out Civ5 and be bored. Which is the case for me. Of course, we can also try to go from subjective feelings to objective facts, as you did in the rest of your post. But I actually disagree with that one as well.
In what ways does Civ5 fail to hold up to those games (ignoring Civ4 since there are 4 billion threads on it)? I've played all of them, and while the older games were good in their day and fun to come back to sometimes, they have all kinds of glaring flaws.
Yep, they do have their flaws. Each of the games I mentioned also managed to outshine its flaws though. I'm not sure whether Civ5 can manage that (though it's not impossible, but see below).
SMAC AI is easily exploitable (it never got UN or trades), combat is extremely bland in spite of a ton of unit options, and for as long as the tech tree is there aren't a whole lot of decisions.
SMAC is a game with a great setting and an enticing story. It lets you terraform a whole planet, fight or use native life forms, perform psi battle in addition to regular troop types, has espionage, a flexible system of government policies, and even multilateral diplomacy (though rather rudimentary). Civ5 has no terraforming, no espionage, has only regular troop types except one that's absolutely ridculous, has no multilateral diplomacy except a broken victory condition, and robs the rivals of their character by making talking to them useless since they "play to win". I won't even think about the artistic quality and creativity of some of SMAC's vctory movies because in comparison to Civ5's wonder screens it makes me cry (just show these to someone who doesn't know anything about computer gaming and ask them which one is from 1999 and which from 2010, the movie or the still screen). I don't know what you mean by "technology decisions" as it's pretty clear that the game wasn't designed to allow you to pick specific techs (this option was added as an afterthought), I don't really miss it though.
You're picking out the one most evident flaw that SMAC has (an AI that can't play the game) and ignore everything else. The ironic thing about that is that Civ5's AI can't play the game either, it fails at the new centerpiece of gameplay, tactical combat. The difference is that SMAC still has its great setting to offer, and all the things that it allows the player to do, while Civ5 in its current state is just a wargame with a broken core mechanic.
Basically, SMAC is a strategy game that gave the player a lot to do and developed the AI as an afterthought. That's why the design still holds up, its core holds well together. The broken AI hurts Civ5 much more than it hurts SMAC.
MOO has vastly less colony management options with vastly more annoying MM, has nothing to counter ICS, very little technology development decisions, and tactical AI is a joke.
I was talking about MoO. You seem to be talking about MoO2. MoO actually is (imho) a masterpiece precisely because it totally
avoids any unnecessary micromanagement.
MOM doesn't have much in the way of city development either, and the AI can't handle some very simple strats (IIRC going for then building paladins is pretty much an auto win).
MoM has 214 spells with many unique effects to explore, about 200 troop types, more than 100 artifacts, 35 heroes with partly fixed, partly random attributes, 14 distinct races, lots of unique effects (breath attacks, death gaze, doom gaze, stoning gaze or touch, life-stealing attacks which even create undead, etc.), and lots more. It's similar to SMAC in that it gives the player a lot of possibilities but has a weak AI. Again, the difference to Civ5 is that in MoM the sheer amount of content outshines the AI's problems in using it.
You can pick flaws in any game, none is perfect. There are many games which succumbed under their flaws (many have been mentioned in these threads). Others outshine them due to other strengths. SMAC, MoO and MoM do. I'm currently not sure whether Civ5 can.