Horizons was dying in its final years, but it only needed an update. Throughout its lifetime, the ride's massive capacity made it seem like it wasn't pulling a crowd, when in fact it handled crowds well. POTC and HM were similarly efficient before FP+ destroyed their hourly capacities.
An "update" wouldn't have cured the general lack of interest/excitement the general guest felt about Horizons. Yes, it had high capacity, but it wasn't a draw. And FP+ didn't "destroy" the hourly capacity of HM and PotC. The hourly capacity is exactly the same now as it was before. The MK has simply been more crowded (especially this summer as it was basically holiday-sized crowds almost every day), which is (by far) the most significant factor in longer wait times. Heck, my first trip to WDW in 1994, before FP even existed, the wait time for Splash Mountain midday was 150 minutes, and Space was 120. I wonder what people blamed for that back then without FP to be a scapegoat.
Standby waits might seem slightly longer because the line moves slower because of FP/FP+, but the fact remains that if 20,000 people can ride HM in a day, then it's 20,000 people whether there's FP+ or no FP at all. The ride still loads the same number of guests per hour regardless.
IMHO, Horizons, WOM, and Imagination suffered from apparent sameness. The rides were extremely different, yet they were also long omnimovers full of AAs. Today, it's difficult to imagine fans complaining about that
No, "fans" wouldn't complain about that, but most guests wouldn't ride those attractions, either.
Look, I flippin' adored Horizons and WoM, but they were
dead. No amount of "updates" or refurbs that kept the basic attraction in-tact was going to make people suddenly flock to them, nor was it going to make sponsors suddenly interested again. Carousel of Progress is in that same category and it's barely holding on by a
thread, mostly because it's a Walt-created attraction from the 60's, while WoM and Horizons were built in the 80s and didn't have the historical status that CoP does.
Spaceship Earth would probably be dead too if not for its location (people are nearly forced to ride it because of it's placement and sheer size; everyone wants to ride inside the "big golf ball").