let's say I ran out to democracy to build the SoL and run US/Eman; I nabbed communism for the Kremlin. Then I tech out to combustion for destroyers and cre con. My AI target has railroad and assembly line. I could actually be "ahead" in overall tech here but lack infantry and airships. Even if I did have airships, what dictates that I have an airbase in range to attack?
Nothing dicates anything, although I rarely spend beakers on democracy nor hammers on SoL. What I'm gunning for wonder-wise are Cristo for a free Spiritual trait and Internet for free techs. Everything else is gravy, "nice" to have. AIs will trade out democracy eventually, before the Emancipation nuke becomes too damaging and combustion and physics ARE along the way to radio and computers. And along that way I pick up assembly line and industrialism, so, "oh by the way Mr. AI, I now have tanks. HA!" With radio, one of my backfill trade priorities becomes flight (often an aggressive AI will have it) and the non-aggs are just about giving democracy away by then.
In fact scientific method you can usually trade for a bargain--one of the few techs you can have at a trade advantage. I think this is because the AIs want to nuke your monasteries. Fine, nuke 'em, gimme almost free tech plz, kthxbye.
3:1 is not good enough vs machine guns/infantry. They will be drilled and they take no collateral. My CRIII rifle will be utterly hosed by even a CG II infantry; and let's be honest here hordes of CRIII rifles aren't exactly cheap and the benefit is not that large given the expected burn rate against even completely toasted infantry/machine guns.
Drilled take "less" collateral, not "no" collateral. And we're talking the AI here not a smart human player who knows how to defend. Agg AIs will go the full CG line and the full C line, and Protectives are the only big worry here, but they usually only keep DI and follow CG up to III. After my airships can't do anymore damage, then it's safe to where I won't lose more than 1 or 2 cannons. And after that bombardment round is done, it's safe for just about anything reasonably strong to attack. I've even had cuirassiers own infs at this point.
And thanks to the AI propensity to pack in cities at the minimum 3 tiles away from each other, a wide-front modern war gives me plenty of airbases even if I can't pick forts to launch from, or friendly AI cities. If they have me at a chokepoint the war tends to be more... costly.