There are 2 things a theme park can do when a rides get beyond repairs;
1) scrap it.
2) update/rebuild it.
Both have the same cost. The company checks the t-shirt sales, ride photo's and other income (hey Moore84 when did you buy from that MIB souvenir store?). Almost always it's better to build something new (easier to maintain, higher reliability, safer, more energy sufficient running costs and they can promote a NEW ride). But what about Hulk, you will ask, that is because of the rodent down the road who forces Universal to keep the land as is if they want to keep the ip rights (they checked sales in the stores and their hatred of Disney so it was an easy decision).
But what about the mouse park you will ask, well they let their business decisions be dictated by their most vocal guests. They would get a tantrum when the horror that is It's a Small World gets axed and that's why we don't get It's a Booze World or other fun rides in that location.
I don't say it sometimes is disappointing that a ride leaves that you enjoyed for 20 years but in most cases the replacement is worth the change. Take Disaster, that attraction was 90% boring (15 minutes) and 10% (2 minutes) amazing. Now it's only 5 minutes boring and it's over, that is a win right.