The Gargano vs. Ciampa storyline was maybe the best example of modern long form wrestling storytelling. The feud stretched from TakeOver Chicago in 2017, and was written to blowoff in a World Title match over WrestleMania weekend, 2019. Of course, Ciampa’s injury costs us the blowoff.
Are there any other feuds this long running and good, that failed to have a proper blowoff whether due to injury or someone leaving a company???
Brutus Beefcake v. Mr. Perfect in 1990 was clearly leading to Brutus winning the belt, although that wasn't like a classic build or anything. But there was definitely a long term story being told there.
Obviously Rick Rude v. Big Bossman is another one where the departure of one guy wrecked the whole program.
There's tons of more but those two are off the top of my head.