This points to a general logic error in the way this works, at least to me. Because units become obsolete, it's possible to miss out on important eurekas or other achievement milestones because you got a tech that made something obsolete. (I don't play Babylon much, but I would imagine it's a huge problem for them.) A similar problem is having formed corps or armies out of units that you need to promote a certain number of.
If they were willing to patch this, it would be pretty simple:
Don't make units obsolete if they are required for outstanding eurekas.
OR
Count promoted units as the more archaic units for the purpose of eurekas.
Also, count corps as 2 units and armies as 3 for the purpose of eurekas.