Casey Jr. is a train ride if I'm not mistaken - not a coaster. That being said, Disneyland has a larger crop of attractions than Magic Kingdom in Disney World, and after 2012 if WDW remains the same (unlikely) the Disneyland Resort will be rapidly approaching the total number of attractions in all 4 WDW parks.
Manta has been heralded as a fantastic coaster - has it taken away from Disney's attendance? Universal has missed the boat on Rip Ride Rockit as they haven't been able to get it open for the summer crowds. My guess is that Universal has been taking the hit because of Manta, not Disney.
To me, the problem isn't necessarily adding thrill rides - sure, I like the thrills as much as the next guy, but what needs to be added is capacity. The Fantasyland expansion will accomplish this in the Magic Kingdom. Truth be told, all of the parks need an increase in capacity. Obviously adding high capacity D and E tickets to any of the parks would help lengthen the day, lengthen the trip and improve the guest experience.
If we're to believe the 5 year expansion plan for the MK (Jim "Grain of Salt" Hill), we're supposed to see phased construction that would start with the Beauty and the Beast area, then Little Mermaid (opening 6 months after DCA's version), and then the Enchanted Forest area that would include several smaller attractions (my hope is B/C level dark rides, but likely smaller stage shows and what not). This type of expansion would dramatically increase the overall capacity of the Magic Kingdom. My hope is that this, along with Harry Potter will see the other Disney parks attendance suffer, causing Disney to add new things across the board.