I'm personally not a fan of uniting envoys, governor titles, and spies into one currency, at least not in the way they're in the game currently. I wouldn't mind if there were some type of trade-off that existed between them, so that a player could potentially focus on being particularly strong at one at the expense of the other 2; however I think it should be possible to play an especially diplomatic game where your civilization is strong in all 3, as well as an especially undiplomatic one where your civilization is weak in all 3. Right now there isn't enough variation from game to game with respect to any of them.
To me, the issue isn't that we have too many diplomatic currencies, but that these diplomatic currencies' current implementations aren't all that interesting. Let's take envoys, since that's actually the most interesting of the 3. If I want to play a game that maximizes my envoys, what should I do?
1. Explore aggressively to try to be the first civ to meet as many city-states as possible.
2. Build the Apadana, then spam wonders in the capital.
3. Build Kilwa Kisiwani.
4. Complete as many city-state quests as possible.
5. Adopt the next tier of government as soon as possible.
6. Run Monarchy as T2 government.
7. Rush the civics that award envoys.
8. Run the Charismatic Leader policy card, then the Gunboat Diplomacy policy card.
9. Run the Diplomatic League policy card until you have an envoy at every city-state.
10. Play as Greece, and build an Acropolis in every city as soon as possible. (I'd say play as Georgia and convert all the city-states to your religion, but as we all know, no list for optimizing anything in Civ VI includes "play as Georgia.")
At first glance, that looks like a bunch of ways for the player to actively impact the number of envoys they get! But let's look a little closer.
1. On lower difficulties, I'm going to explore as much as possible regardless of my envoy strategy, or lack thereof. If I'm playing on Deity, I won't be able to focus on exploration either way.
2. Again, on lower difficulties I'll often build the Apadana and a bunch of wonders in my capital either way. On Deity, I'm not getting the Apadana.
3. See #2, but with Kilwa.
4. Most city-state quests fall into either things I'm going to do anyway (Build a Campus, Send a Trade Route) or things I won't do just to get an envoy even if I really want one (Recruit a Great General, Religious Conversion when I haven't founded a religion).
5. I'm getting my next government ASAP every game no matter my strategy.
6. Hey look, a legitimate decision with trade-offs that can increase envoys. Neat!
7. I suppose you can shave a few turns off acquiring envoys if you really focus on the relevant civics, but it won't change the total number available.
8. Given that you get a diplomatic slot in the early game with only Charismatic Leader and Diplomatic League available to fill it, you're going to be doing some of this no matter what.
9. See #8.
10. Certainly changes up the envoy game; also inherently irrelevant to the vast majority of games.
In the end, my experience is that I play basically two kinds of games in Civ VI with respect to envoys: games at lower difficulty, where I'm basically always suzerain of all of the city-states, even the ones I don't really want, and games at higher difficulty, where, to the extent the city-states don't all get conquered, I have trouble maintaining suzerainty of a city-state even if I focus on doing so. Spies and governor titles, which have even fewer ways for the human player to impact their total number, suffer from the same problem but worse.
I don't think the solution to this is to combine envoys with spies and governor titles, but flesh out city-state diplomacy so it's actually engaging. Getting envoys from liberating a city-state is great. It would be even better if I could get them by defending a city-state before it gets conquered. Or by gifting the city-state units. Or by using my builder chargers to improve its lands. Or by having a trade route or sharing a religion (there's no reason this should be limited to a couple city-states in a specific era.) For that matter, I don't understand why city-state diplomacy is limited to essentially a snapshot of the number of envoys at a particular moment; I've been allied with my neighbor Hattusa for 5 millennia, and they desert me overnight because some other civ from across an ocean stuck an Amani in there? How amazing at diplomacy is she supposed to be?!
For someone who already thinks the game is too complicated, envoys could still be safely ignored if they make changes like these; for someone who wants to pay attention to city-state diplomacy, there might actually be real, substantive differences depending on strategy. Everybody wins.