Not really a complaint, just legitimately curious. Aside from Pyke's ult which triggers only on kills, I don't believe any other support champion has a similar mechanic where they get gold for performing their little minigame.
Why does Senna get free gold for playing her minigame?
YOU gain gold from playing the minigames Gangplank/Zyra/Yorick/Malhazar present you as an opponent.
Twisted fate gets a free 1-6 Gold per minion he kills (ALL MINIONS).
Draven's passive literally is a massive trove of self earning gold. (Huge reason people perceive him broken.
It's not like Senna's gold gain is anything like ANY of the above. It's about 90-180g a game as a support. It's about 75-150g a game as an adc.
Furthermore, she literally has 11-24 second cooldowns, so she doesn't have normal poke rotations as a support compared to others who have 5-16 second scaling. Once they add post 1000g gold back to the game, you're going to see a difference in how much gold Senna gets vs how much other supports with clear do.
It's mostly because it's comparable to Illaoi, GP, Zyra, Yorick, Malz, and wards.
It's a small amount of gold to reward doing something, which Riot commonly does.
Lets math this shit!
Each wraith gives 1 AD, which is equal to 35 gold when compared to the cost of 1 
(+10 AD for 350g)
For every 20 wraiths, you get an additional 15% critical strike chance. Compared to 25% critical strike chance with a value of 800 gold, 20 stacks of Senna's passive gives an added value of 600 gold.
600 / 20 = 30g
Attack range is a difficult stat to give a gold value as there isn't a way to directly purchase it other than with rapidfire cannon, which doesn't give a range value on the lolwiki page. However, not accounting for the passive RFC is only 67% cost effective. So if the passive is considered to be the remainder of the gold value, it can be assumed that 35% of Senna's attack range is equal to 32.79% of RFC's total cost.
0.35 (600) = 210 range 0.3279 ( 2600g) = 852.4 gold
It takes 9 x 20 wraiths for Senna to exceed the range provided by RFC 180 = 225 range
So theoretically the gold value this range can be divided by the number of stacks to give the gold value of 20 stacks, or each stack, regarding range 854.2 / 9 = 94.9g 852.4 / 180 = 4.75g
So at a minimum, every wraith gives Senna 38 gold value (+3 flat gold +35 gold value of AD)
At 20 wraiths she gains an additional 600 (+15% critical strike chance) and 94.9 (+25 attack range) gold value on top of the 90 flat gold and 700 (+20 attack damage) gold value. That's 1454.9 gold per 20 wraiths.
If we were to just give her this value per stack and not work on the 20 wraith milestones, that's roughly 72 gold per wraith
Marksmen typically scale better with gold than other classes. As a support, you typically have less gold to work with because you aren't farming. The extra gold is designed to compensate for this.
Similar with Pyke. He isn't a marksman, but Assassins are budgeted around solo lane gold. .. Really, everyone tends to get more gold than supports. So when supports need to be built similarly to other roles, they need more gold. Tank and utility supports don't need as much gold because tank / utility items are cheaper and the champion typically has higher base stats to compensate.
The issue is, these new support items have actually helped the support budget A LOT. I am playing and seeing more supports played with full damage builds simply because they can afford it. Personally, (as a support main) I think this is a good change, but I think Riot wont allow it to continue like this. I feel like I can be an actual champion by the end of the game, even if my carry made some dumb plays. Before, if my carry falls behind and I'm already on a budget, I am worthless unless I decide to just start taking their farm.
Also, as far as Senna specifically goes... They made a good decision (in my opinion) in how they balanced wraith drop rate.
Small minions and lesser monsters have a 20% chance to spawn Mist Wraiths, reduced to 1.67% chance on those that Senna kills.
So if she is getting the gold from farming, she is very unlikely to also get gold from wraiths. If she is farming, she can afford more expensive items, giving her better stats from items. On the other hand, if she isn't farming, she is more likely to get better stats from her passive, but still gets some help with her ability to purchase items.
It is 3g each one..
In a normal match you are lucky to get to 100 stacks of them.. even then some of those are 2 stacks of the buff for big mobs and players..
So at most it is a 300 gold difference.
Yay a free pair of boots or a starter item!!!!
If you are worried about bot lane shared income you should be looking at pyke instead. His execute gives 600+ team gold.
