I have a love/hate relationship with the Terror Tram as well. Like most of you, I agree that it's basically the same thing nearly every year, just different costumes and small set design changes. I wish they could vary the pathway more, and/or include more indoor sections (thought that always creates bottlenecks, it's own problem).... not to mention more hiding spaces for the scareactors.
That said, it's something not only very unique to Hollywood HHN, but unique to the haunt scene in general. The only other haunt experience that even comes close is maybe the Haunted Hayride. but I'm not even sure that's a true equal comparison. I find myself usually enjoying Terror Tram (for what it is) despite it's obvious flaws. I think it's the energy levels of the crowds and scareactors that does it for me. I do get a thrill from people watching, and aside from Scare Zones (where it does feel a bit awkward to stand there and gawk), there's no better place to do it.
I also think we forget that most people don't go to HHN year after year, like many of us do. So for a lot of people, every year there are a huge number of guests where it is their first time. It still amazes me that when they get to the drop off point and the host says, "Looks like you'll have to get out and walk," there's nearly always 1/3 of the tram screams in surprise, "Awww, hell no!" And I think we have to admit, even for us veterans, the very first time we did this, it was insane.
I did do the Stranger Things Drive Through experience, and I do think something like that could be done with Terror Tram (and would be interesting!), but I also think Universal doesn't really want to reinvent the wheel trying something so drastically different, when their current formula still basically works.
Side note: I heard one of the reasons Jupiter's Claim was cut from the Tram was that there were too many people stealing and damaging props from the sets. Reducing the crowd to VIP levels would better allow them to supervise guest behavior. I don't know if that's true, but it sadly sounds very believable to me.