Arecibo was also a massively powerful radar transmitter, used to map other bodies in our solar system and arguably more importantly to get precise position and trajectory information for the various objects floating around out there. Losing it is a massive hit to our ability to accurately track potential asteroid threats.
Now we're down to just Goldstone, which is both much less powerful and has a much smaller dish.
You may know two people who have no use for Arecibo, but I know of many many more people who know the actual capabilities of Arecibo and know just how impactful the loss is, and that's just on the science side. The telescope also served as a gateway and source of inspiration for many young students in Puerto Rico. The Observatory ran many STEM related outreach programs over the years, with far reaching impacts.
The entire field of Planetary Astronomy has lost one of the best instruments for Earth-based observation of planetary objects in our solar system. Additionally, our capabilities for observing near-Earth asteroids has been been diminished, with Goldstone being the next most capable though it's sensitivity is nearly 20x lower than that of Arecibo.
FAST can perform some of the same science Arecibo can, but there's still a fair amount that can't. It's frequency range only goes up to 3GHz, which is only a third of what Arecibo's range was. Off-zenith, it's illumination area was only 25m in diameter larger (off-zenith it's beam was only 300m compared to Arecibo's 275m) so for most cases it's gain factor isn't substantially larger that Arecibo. Additionally, it's focal platform is much smaller, leading to less flexibility, though it's reduced frequency range does mean it needs fewer receivers to cover that range.
I think characterizing Arecibo as a microwave observatory is not correct. While AO did observe between 3 & 10GHz, that's only a tiny portion of the Microwave spectrum. AO's design was adequate for it's purpose, though clearly a newer redesign would make plenty of sense.
A white elephant it was not. It had modest budget needs compared to many other facilities. It's base operating costs was around 10 Million USD. For more serious maintenance efforts, it needed supplemental funds. However, it's been operating below that since NSF Astronomy started dialing back funding in the late 2000s.
NASA and NSF Atmospherics stepped in with additional funding as NSF Astro stepped back, but that still just met the operating costs and left little on the table for larger maintenance efforts.
Like many have said, the time for major maintenance was a decade ago, however, the funds weren't there, and neither was the knowledge that anything was wrong with the primary cables. All inspections performed showed no indication of major problems. We now know that the cables were not performing to the designed ratings, but hindsight is always 20/20.
Same as the Space Shuttle which spent decades siphoning off a good chunk of of the budget for space stuff.
It was white elephant.