Most large attractions are designed to be there essentially forever. An attractions lifespan actually comes down to how well it is received by the public, its attendance and its cost to operate. Once an attraction no longer pulls it's weight the decision has to be made on what to do with it. Sometimes updating an attraction can spark enough interest to save an attraction. In more extreme cases, closing or replacing are the best options.
Attractions like IASW, Pooh, Peter Pan, tea cups, etc remain largely unchanged and will more than likely never go away because they are inexpensive to operate and almost always operate at near full capacity. PotC and HM receive occasional updates to help keep them fresh. 20k was a much loved attraction and operated at or near capacity, but it had very low capacity numbers and cost a rather tidy sum to operate and was shuttered as a result. Horizons and WoM had incredible capacity numbers, but they were very poorly attended in that latter years. While an update might have given both attractions a little more life the decision was made to cut bait and run and simply replace both attractions.