I think
@Rick put it perfectly; public support of SeaWorld's former star attraction has been on the decline for a while now, and that will only get worse as we move into the future. Trust me, knowing people my age, the general opinion amongst them about SeaWorld's former practices with cetaceans is far from positive. Considering these are the guests of the future, I'd suggest that sticking to their original star attraction would have been a death sentence for them. And SeaWorld as a company does not deserve to die; their rescue & rehabilitation work they do is truly amazing, and they help thousands of animals across the ocean. That work deserves to continue for the good of the natural world.
I know that SeaWorld does lots of amazing conservation and rescue work, as I just mentioned, but I don't think that was ever what the big crowd puller was in the park's early days. If you go back even as recently as to the SeaWorld of the late 2000s-early 2010s, the orcas & their respective shows were the stars of the show, and I think that can probably be implied from the fact that most things in the park had either Shamu's name or some sort of orca plastered on them until very recently. For example; Shamu's Happy Harbour. Dine with Shamu. The Shamu meet and greet in the park's entrance plaza. The entrance feature to the park that still has huge orcas on it now. I could probably think of more, but those are just some that come to mind. My point is; the orca shows were the star attraction that made SeaWorld its name, so when Blackfish, alongside other factors, hugely tarnished the reputation of these shows, SeaWorld naturally had to do things very differently in order to survive. Like the animals it helps to protect, it has had to evolve and adapt to environmental changes as time goes on (to use a topical analogy).
So the path that they've chosen to go down in order to do this is the path of rides. I personally think that this is a very good path to go down for a number of reasons. Firstly, big coasters with impressive stats will definitely pull people in and give people in Orlando a reason to visit. A roller coaster is a very eye-catching thing to be able to put into a marketing campaign, and it's a good way to pull people in. Secondly, they can really differentiate themselves from Disney & Universal in the theme park stakes with the path they've gone down. While Walt Disney World and Universal Orlando both have many great rides and are still building some very eye-catching coasters, I would not call either company a company that focuses hugely on thrill rides by any stretch (I guess you could call Universal thrill-focused to an extent, but to nowhere near the same extent as what SeaWorld now is.). A big coaster-focused park is something that the Orlando area lacks (I know Busch Gardens Tampa exists, but it's probably too far away for your average Orlando visitor to want to visit), so SeaWorld will really be able to promote themselves as something different, and they're really filling a gap in the Orlando market. Combine that with cheaper prices than Universal & Disney (I think?) and the lovely atmosphere that the SeaWorld & Busch parks have and you have yourself a very compelling product to sell!
So basically, before I ramble on for too long; I think this move was necessary in order for SeaWorld to survive in the long-term, and I think it will benefit them hugely in years to come. Besides, as
@MakoMania said, I think the conservation and rescue work will still continue and will still be a popular element of the park; one thing that recent SeaWorld adverts have emphasised heavily is that visiting helps save animals.