RE: Guide: How to Not Be an Annoying Source Material Fan
It depends on why they liked/disliked a show and what their worldview is when it comes to art/entertainment. If they liked/dislike a show for an asinine reason then I'll try and convince them otherwise, but if they like/dislike a show for fundamental reasons I concede that I can't change their mind.
If their worldview is that all art is good and entertainment exists to be enjoyed then I can't change their mind from the idea that if they enjoy it it must be good. However, if the worldview is that the quality of the art and entertainment matters, I can have a discussion with them and try to convince them of my position.
The only exceptions to this are shows that are degenerate. Most commonly, there are elements of pedophilia in them. I will not concede any ground when it comes to such shows because they're a danger for the anime industry and should not be consumed. Eromanga Sensei and Hyperdimension Neptunia come to mind.
While it is a given that one must act about what one finds morally reprehensible, I do find it interesting that your objection to these shows is more that they're "a danger to the industry."
But I do want to make two points on said content: Many people like it because it is taboo, so you telling them it is only reaffirms it for them. Furthermore, researches sadly show that arguing with people only serves to push their opinions farther apart from the center, so it's usually not helping anything :(
The number of people who like such shows are a niche group that would like it anyway. The danger to the industry is that if more of those niche shows are made the potential for mass appeal diminishes. At the moment the industry is extremely inbred and cannot possibly sustain itself this way for much longer. Appealing to niche groups can work with high value goods like cars and motorbikes, but anime is not high value and so cuts off possible markets.
I have found with said degenerate shows that most people are willing to listen or concede that they have problems, but only the vocal fans have objections to calling it out as such. I browse places like /a/ regularly, and while most "lolicons" are just memeing, there's a percentage that are legitimate. Of those, many know that what they're doing is morally reprehensible but do it anyway.