Comparing biodegradable organic waste and plastic are an apple and agent orange comparison. One is costly in terms of cash to separate from the waste water (yeast/trub), the other (plastic) has specific harmful effects in the environment. It is not at all similar to not recycling plastic..

Second, I agree that you shouldn't flush general waste down the toilet but yeast/trub isn't general waste. Cereal boxes are general waste. empty bottles and aluminum cans and plastic bottles and old cooking grease and unused motor oil and household cleaning chemicals..... those are general waste and I will not be flushing them and you shouldn't either. Yeast and trub are specific waste which while they may have a calculable cost to remove from the waste water stream, are not in and of themselves poisonous to the average human, bird, fish, or in the case of OP, tree.

and C. this is still a question of volume and scale. How much trub VS how much other non trub and non excrement waste does the local turd plant have to deal with? How many tons of things like household scouring powder needs to be removed from that water volume? How many tons of plain old dirt washed off our bodies do they have to deal with? Hell, how many tons of dead skin cells and hair need to be removed from the waste water stream?

According to a cursory googling the average human makes about a pound of skin each year. In a city of 100,000 people thats 100,000 pounds or 50 TONS of skin a year. Not all will wash down a drain so if 1% of that skin makes it into a sanitary sewer drain that's half a ton or 1000 pounds of dead skin cells that need to be removed from the sanitary sewer stream before that water can be released back into the environment safely.

How is my gallon of trub gonna even register on that scale?

It's still my hypothesis that it isn't.