How about the Vekoma boomerangs? Those started being made in the 80s I think and many are in rough condition these days.
And how about Looping Starships? And most Enterprises, Pirate Ships, and Round-Ups are getting old too. I think removing these is mostly due to operating and repair costs as people have said, but I also think to a degree it's due to an advancement of technology. Inverted coasters succeeded suspended coasters, and there are so many new interesting flat rides that companies are selling these days that some of the older common ones like I've listed will lose their appeal as people come to expect a more dynamic experience. Cookie-cutter coasters like all the old double corkscrews, boomerangs, and slcs are being replaced with new cookie cutter models like the euro fighters, zacspins, and the spinning wild mice.
At least we can take solace that most of the remaining arrow megaloopers will probably only be removed from their parks to be replaced by something equally cool, like how SUF replaced Shockwave and Green Lantern replaced GASM. I just hope when Loch Ness Monster does finally make it to the scrap pile, it will be because its spot is needed for something new and awesome (and that hopefully has a picturesque interlocking loop over the river).