Weak mobs at 75 had around 60-70 AGI. You needed around 110-120 DEX to cap crit rate. It's pretty fair to assume mobs you'd be fighting at level 90 (Lv. 95-100ish) would have 80-90 AGI (This is a number that most people use when theory crafting). That would mean you would need 130-140AGI to cap crit rate. In a standard Good ranger TP set, that doesn't use either Skote or AF3 feet, you're looking at around 100-110 AGI. You can get that to 130-140ish with pieces like Skotes, AF3 feet, scout's belt, etc, but you're statement of "it shouldn't even be a challenge" is wrong. Obtaining the gear is not a challenge, deciding to use pure AGI gear (Af3 feet and skote for example) over better damage gear (Blood Fng. Gnt/AF3 hands depending, and Relic Feet +1) is a challenge.
You, like so many other posters who don't actually look into ger sets and stats enough, would do well to shut up and stop posting false infomation without even a hint of research.
The argument of "There's a lot of AGI on rng gear so it probably affects crit rate" is very weak. There's a lot of Vit on drg gear, even AF3, yet Vit is only useful for Jump (Not High Jump, Super Jump, spirit Jump or Soul Jump... just Jump). As mentioned, Dnc has a lot of gear with chr. Because Chr is useful for healing. Thf has gear with Chr, because of dancing edge. Drk has gear with int and mnd, mnd because it's a mod on guillotine and int because of their elemental magic. Ranger has a lot of AGI gear. All RNG WSs except Jishnu's have AGI mods. AGI increases Racc. These two are reason enough to put AGI on a lot of RNG gear. It's not reason enough to argue that AGI affects crit rate for ranged attacks.
Furthermore, you say "Rng gear has lots of agi but little dex". That is not accurate either. Yes, artifact, relic and empyrean maybe, however most of ranger's other gear is shared by other light armor jobs like cor, thf, dnc, etc and generally has a good amount of dex on too. Aurore, Skadi, Denali, etc.
It really is a simple test to do. Go out for a merit alliance in abyssea. Fight a decent number of monsters with an atma which has neither crit rate, dex or agi on. Fight the same amount of monsters with atma of the kirin (agi+) then fight same again with a high dex atma (omnipotent). Parse resullts. It should be fairly obvious as one should raise your crit rate by at least 10%. Fight strong enough (IT dom op) monsters so that cruor buffs alone aren't enough to cap dDex (or dAGI, whichever it turns out to be).
The only reasonable argument so far for AGI affecting ranged crit rate is "It;s more logical". To that Iw ould reply "Hi, are you new to FFXI? Are you new to SquareEnix?" Logic means jack diddly. This is SE we're talking about.
You may have done a test but if you can't provide data then it is meaningless. I could come and say "I've done thousands of tests with CHR and enmity increase and decrease and noticed that CHR does affect enmity." But if i dont have proof to back pup these claims then they are completely worthless.