That is an assumption. The AI in my modded game is already able to equal me in research and economy. The ones that I have comprehensive unit designs even challenge me in battle. I can still win, but the advantages are things that are very possible for the AI to learn. Have you ever played GalCiv2:TA? It had the best AI I have ever seen. I still lose sometimes to that AI after I modded out all the cheese. I still have nightmares about those damn Drengin. Seeing as this is the same AI programmer and the game is even more flexible, there is no reason to assert that the AI will not be at least at the same level.
That's not an asumption, if a game is more complex than go or poker, a computer cannot win against a good player currently. I have not tried your mod yet, but I am just finishing a game in Ridiculous in which I wiped the floor with the AI. I have not played Gal Civ 2 that much, as I didn't really like the gameplay (too much like Civ in Space for my taste). I'll give it another try. But the need to jungle between civilization and heroes make the work of the AI much harder than in Galciv. If the game has enough randomness, the AI can still win sometimes despite being inferior (as in Armageddon Empires), but it cannot consistenly win on a regular basis.
Optimizing the tech tree is relatively easy (it is only a problem of optimizing bonuses). The hard part for the AI is understanding positioning, and the combination of special abilities, to know where and when to attack, such things... If you are into going for a tech race against the AI, you might not be playing to your strength 
I would be interested in trying your mod, though.
Concerning the bigger multiplayer vs single player issue, the percentage of players playing multi is not that relevant : for instance, I only play about 5% of my games in multi, but I much prefer them to have MP, for the following reason:
if the game is really good, you can get the most of the gameplay if the game includes MP, while on the other hand, if it is SP only, you know you'll get to a point where there is nothing left to do with the game (except mod the hell out of it, but I am not patient enough for this).
More often than not, if I don't play MP, it is because I don't like the game enough to spend dozens of hours to play it in MP, or because I don't think the game would play well in multi at all.