I'm surprised that a asym spin is faster on a Capri 30 than a sym spin. In a good breeze it seems like it would be faster to go DDW then to heat it up, and the Capri 30 seems unlikely to plane (making good use of apparent wind).
I'd like to hear the rigging that people are using for asym kites on poles when racing. I do it when cruising, but I'm pretty slow about jybing (it's a lot faster for me to jybe the sym kite on the pole). The only way to make it fast seems complicated: 2 guys and a tack line all attached to the tack of the kite. Blow the pole, jybe the kite flying it off of the tack line, then bring the kite back onto the pole and use the new guy. I basically do the same thing when cruising, but normally just keep one guy attached and re-run it to the other side of the boat. It's a lot more lines than flying the sym kite on my 28' boat (where I can use the same lines as spin guys and sheets on a symmetric kite).
My cruising asym is a lot smaller (550sqft) than my symmetric kite (700sqft), so I can't make any useful speed comparisons.
I'm thinking about a larger (35ish) performance oriented boat as an upgrade in a year or two and a sprit boat like a J/109 makes a lot of sense for short handed sailing. I'm not convinced that they are faster downwind than the symmetric spinnaker boats with similar ratings though (like the J/35).
It may on a reach, it won't on a vmg run. What you stated is true, the C30 will not plane nor move the apparent forward significantly enough to beat it's sistership w a sym kite on a run.
Back to the OP's question, will the Sym's die a slow death? again my answer is no. There will always be an educated sailing population, as long as the handicapped version of our sport exists, who understands the performance difference of the Sym kite. That alone will keep her alive...not to mention the many OD sym boats who will never change.