I agree about Dorian's quest, but I think the only thing his quest reduces is resources that could have been better spent on giving him a much better story line. And I feel this way about all of the companions. Their stories are usually side stuff that is irrelevant to the main plot. I wouldn't mind it if resources were spent integrating them more into the main story. I don't get how adding "trending issues" takes anything away from the plot. In ME1 a female Shep could romance Liara, in DAO, a male can romance Zevran and a female can romance Leliana. So what is so different now? I think you are confusing the addition of "trending issues" (additions BW always included) to a deterioration in their writing dept. One thing has nothing at all to do with the other. The writing has simply gotten bad. The problem isn't that Dorian is gay, but that the writing for it was just terrible and even screwed around with the settings own lore to make it happen tot he point where it comes across as a bit convoluted. And Dorian isn't the only companion with some terribad writing. And ME3 would have been written the same way even without Steve and Traynor and even if they never made Kaidan a gay LI. As LIs are nothing more than side content. How many players would have chosen a Hanar doc over Chakwas to justify the resources spent? I do agree that it would have been nice if we could have learned more about the other races, but that's more of a flaw in the game's overall story than LGBT stuff. The narrative never gave us time to actually travel and meet different Elcors et al. Like the rescue mission to save Elcor from their home world was just a "send a probe" to the planet deal.
I agree that the writing has gotten bad, but I think you are casting blame on the wrong thing.
I will admit that I opine for the days when Chris L'Etoile was still with BioWare, and you are right, the quality of writing has gone down overall (IMO). Still though, there is a direct correlation between the amount of the non-human elements and the amount of the social topics, regardless of the decline in the writing department. I'm not saying that this is something exclusive to the LGTB elements of the games; the straight elements get in the way just as much.
I understand that such real world issues are BioWare's 'thing', but why is it that the ratio of those elements are increasing while the non-human elements are steadily decreasing with each subsequent title? If anything the division resources regarding world building and the topics of the day should remain consistent, but they don't as far as experience has shown us.
Take DA:I for instance: we get Dorian's personal quest, Krem's airing of baggage about being trans-gendered, the retconning of the Qunari to accept such things, vilifying Tevinter further by making their ruling class largely homophobic, the card game scene meant to foster camaraderie, Cassandra's questioning of her faith etc. But not one mention of Werewovles or Sylvans (outside of a very stupid Plants vs. Zombies reference in an obscure lore entry), the complete ignoring of any mention of the events during Awakening that could have massive ramifications on the plot, no further exploration of the fear demon that has potentially been feeding off of Thedas for eons, the reduction of Darkspawn and dragons back to mindless, stupid monsters, etc.
There is nothing wrong with the game having a character who: likes boys, or feels bad because their father never loved them as a child, or sees themselves as the opposite gender than the one they were born with, etc. It's just when the narrative actively decides to place those elements as the focus over letting the player go over and converse with the Rachni Queen more in depth, or find out about the Awakened more (for example) that it starts to get obnoxious. We get it, you want to talk about such things BioWare, but does the universe have to shrink to accommodate that stuff? Does the level of diversity that the first title had have to be removed so that we can get more discussions about trending cultural topics?