I 'm not a writer, but I know some basics.
Everything you write in a story, should have some purpose. It should contribute to something. World building, character development...
The full development of this love story have 0 purpose. The same message to Joel could have been done in 15 or 20 minutes, and it would make no difference to the rest of the story.
But obviously, Druckmann and HBO couldnt miss the opportunity to make something big out of this gay relationship. I would bet my neck that if the couple were straight, there would be no filler, because
then noone would be talking about it. Straigh couples dont sell. LGTBQI+ does. That's the only reason this episode is like it is.
I mean, for sure, it was a pretty gay episode. Whether you view that through the lens of trying to tell a contemporary story and factoring in that, yes, that are homosexuals in the world, or you view it through the lens of pushing identity politics in a Trojan horse plowing straight into the center of BelovedIPville, the fact of the matter still stands: this is an adaptation, and it's most compelling thus far (subjectively speaking, of course) when it adds context through new scenes or twists established narrative points to weave a more enthralling piece of passive media.
At the end of the day, let's take a step back and realize that no one single opinion or viewpoint in interpreting art can be taken as empirically correct, and simultaneously, more than one thing can be true at a time. Was *this* story necessary to further the main storyline of Joel and Ellie? No. Did it add anything to the main characters' lore? Other than explaining how Bill knew Tess and Joel, not really, no. Was it an episode spent on a side character who is (more than likely) not going to be seen again? Yes. Was it pointless? Again, subjective, but I'd argue that a franchise like The Last of Us should follow similar broadstrokes to some of it's other siblings in Resident Evil and Silent Hill. Not for anything superficial or aesthetic, no. But what these two franchises do is treat their setting as their own main character. Spencer Mansion, Silent Hill, these are fascinating locations dripping with lore and history. The Last of Us's world is a worthy successor to that legacy, I think, in the wealth of potential stories there are to tell in the world, irrespective of the presence or absence of Joel and/or Ellie.
Someone earlier mentioned Bill being a classic foil to Joel, mirroring consequences and benefits that Joel may come to face if he keeps on a similar route, for better or worse. It's roundabout, and kind of makes me think of storytelling that wouldn't be absurd in Japanese media, but I think it's absolutely skillful and a pleasure to see when an artist is good enough at their craft that they can stealthily develop the main characters in a story that doesn't even include them, barring five minutes or so.