Questdog
Prince
For the same reason missionary/inquisitors are more and more expensive. The efficiency of an envoy does not significantly decrease over time. However, its hammer cost become more and more insignificant.
This is not rubber banding, this is era normalisation.
Alternative solutions would be:
1) Have the hammer cost of every diplomatic unit increase with era. (Currently simulated by the those multiple units less and less cost-effectives)
2) Have the influence from quest drastically increase with era, so that quest can compete with "diplomat spam"
Note that this apply to everything. A mechanised infantry in late game is not much more efficient than a spearmen in early game, and relatively to their era hammer production, they have approximately the same cost. But in absolute cost, the infantry is far more expensive than the spearmen.
It is a BIG change, though, from Envoys to Diplomats. 2 paper AND 3.5 times the hammers! It literally shuts down the assembly line.... There is no Renaissance unit to buffer the change, either. I play on Marathon and when it goes from 5 turns to 17 turns to build one, that hurts.