I'd like to point that the envoy system is not as pointless as it might seems. Currently, the three main improvement I see compared to Vanilla "buy CS with money" are:
1) You have a time delay between using your gold to buy Diplo unit and the moment it takes effect. Making "buying CS just before the vote" less exploitable.
2) Since paper is limiting, you can't just spam diplomatic missions in one turn, as you have to wait for the units to reach their destination before continuing buying units. It gives more time for opponent to react and start spamming diplomatic missions to counter you.
3) What I feel is the most important point here: it makes relationship with nearby CS easier than relationship with CS at the other side of the world.
[Note that if we were on a drawing board to rethink diplomatic units, I would vote for them working as "trade routes" that exchange paper and GPT against influence per turn. But that would be a lot of changes and balances issues involved...]