Regardless of whether this is a good idea or not, but Relic has already stated a bunch of times that they do not want players to be able to build both T3 and T4 in a game. The idea is you're supposed to supplement each tiers weakness with commander abilities.
This is the problem, I think Relic thought that having two factions with the same teching structure would mean bad variety (Sov and Osth from release). However if we sacrificed a little variety for better balance I think that it would be terrific. Make the tier's affordable and balance units within the tiers accordingly, and this would make a more enjoyable game+meta.