Why must it be replaced by a coaster? Hypersonic wasn't replaced by a coaster, nor was Shockwave. In fact, I'd consider the replacement of both to be superior from several perspectives.
Cedar Fair has proven since its purchase of KD that they have a vision of Quality>Quantity. Volcano's removal puts the park at 11 roller coasters. While KD has been removing coasters, they have replaced each and every one with something that has positively contributed in making KD a more well rounded park. In many aspects, KD is a much better park today than it was 12 years ago. Adding rides that have more longevity. They've made additions that can please the entire family. KD has spent tons of money on visualization of the park. What was once do considered a "trashy" park is now looked at as something far greater. It doesn't take a roller coaster to make a park better/succeed.
Safari Village is now the most outdated area of the park. (Old Virginia gets a pass ?) Volcano being removed gives Cedar Fair a chance in updating this borderline ugly Paramount abomination. Crypt is barely hanging on at this point, but once Crypt is gone, only a single flat ride will reside in Safari Village. That is a 1:5 flat ride to roller coaster ratio. While it has been of a smaller scale, Cedar Fair has shown willingness to invest in Kings Dominion. The removal of Volcano means more than just a potential new ride, but also the chance of a needed revitalization to an otherwise ugly section of the park.
Removing any icon will result in anger amongst many. When Carowinds announced the removal of Thunder Road, madness pursued and still lingers around from time to time. Fury 325 was built that same year, yet Carowinds managed to maintain record attendance every year after replacing Thunder Road with a water park expansion! Just recent proof that a coaster does not have to be replaced by another coaster to in order for a park to remain successful.
Not relevant to this convo, but it wasn't too long ago that a certain park in Canada added a Dark ride/roller coaster combination built into an existing mountain.
EDIT: Added a Carowinds comparison.