Making the T34-85 a T3 immediately would basically rule the T34-76 at HQ useless, it would turn into another t70. There has to be some sort of fuel cost to gain access to the 85.
Part of the suggestion also included making the T34/76 more of an AI unit rather. That would give it more distinction from the T34/85. The T70 would be lighter and come out earlier than the T34/76 and the T34/85 would come out a little later and be AT.
Making the 85 an upgraded 76 from T3 means that T4 has no access to it and just makes T3 better. T4 currently is a lot less appealing unless it is supplemented by doctrine choice.