Eco Compendium by Cheeseberry
I don't think T2 fabs are far more efficient:
For eco efficiency: T2 mass fabs have the same efficiency as T3 fab grids unless they're placed without shields adjacent to T3 pgens, a template which is very vulnerable to random bombers/arty/novax/tele and not dense at all.
For space efficiency, T3 fabs win by a long shot (1.77x denser, and cleaner templates with pgen adjacency). This also means they are way easier to shield, with high HP on top of that.
For adjacency efficiency, T3 fabs are great next to 2xT3 factories and T2 fabs are great on mex, with roughly equal payback time efficiency. T2 fabs on mex are more common because it's a lower investment cost and factories fit for t3 fab adjacency are rarer than mex with storage.
The differences in payback time are in a matter of seconds so I don't think there is a clear winner for which is better to eco with since the remaining differences are about T3 fab durability vs T2 fab low cost/unit (easier to build a small amount of and start paying back a little quicker).
For the other non-eco points:
I don't think there is a big "consistency" issue with T2 fabs being T2. Lower tech units/structures are often cheaper and more efficient but less dense, and that doesn't mean they become completely obsolete when higher tech appears. Some examples: T1/T2 PD, T2 shields, T1 arty, T1 air scouts, T1 engineers, navy, T1 bombers, T2 gunships, mex upgrades, T3 units (compared to T4s), flak. T2 fabs fit into this pattern just fine.
Making T2 fabs obsolete would be lame and taking out a dimension in how people can eco. Plain downgrade in gameplay.